Princess Catherine‘s appearance at Trooping the Colour may have looked seamless, but it likely took a massive toll on her!

On Monday, royal expert Katie Nicholl spoke with ET about Kate Middleton‘s first official appearance since her cancer diagnosis, which saw her ride in a horse-drawn carriage with her children, Prince George, Princess Charlotte, and Prince Louis, before standing with her whole family on the Buckingham Palace balcony to watch King Charles III‘s birthday parade.

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She wore a white dress with black detailing and kept a smile on her face the whole event — but the insider believes the outing wasn’t as smooth sailing as it appeared. Katie mused:

“The princess has this sort of swan-like quality about her — she’s sort of gliding on the surface — but beneath the water, I think often she’s paddling quite frantically. I can only imagine that a huge amount of preparation and time and effort and energy will have gone into Saturday’s appearance.”

 

The source continued:

“She was on her feet for a long period of time. There was a moment where she was watching the parade… and was offered a seat, which she gratefully accepted. That’s just a bit of an acknowledgement that she is not as strong, not as fit, as she was before she went into this treatment.”

But despite her health challenges, Katie insisted that the Princess of Wales’ attendance was nothing “short of immaculate” and that Kate “looked impeccable,” noting, “If you didn’t know that she was going through cancer treatment, you would never have guessed.” It’s true! But the reality is that the 42-year-old is going through treatment, so this event presumably took “its toll” on her, the expert continued:

“I think a day like Saturday, where she knows the eyes of the world are on her, there is a huge amount of scrutiny that will take its toll. I think we can probably imagine that she probably had a long and well deserved rest afterwards. I don’t think we can expect to see her in public for a little while. That’s going to take some recovery from.”

So, why make the effort in the first place? Not only is Kate truly getting better throughout her ongoing treatment, but she was eager to get out of the house and do something that would bring her joy. Nicholl said attending the parade was the mother of three’s “decision alone,” explaining:

“There was no pressure on her from anyone. She really wanted to do this. She turned a corner in her treatment… When she turned that corner, I was told that Trooping was a goal for her. She was given the sign-off by her medical team. She spoke to her father-in-law about attending and to Prince William. They were fully behind her.”

The monarch, who made an adjustment to the balcony lineup to stand beside his daughter-in-law, was particularly pleased by her attendance, the source shared:

“Charles has been a great pillar of support to Catherine when she said that she’d really like to be there at Trooping. He was 100 percent behind it. The idea that she was going to upstage him didn’t even cross his mind. He just wanted his daughter-in-law by his side.”

Addressing the 75-year-old’s decision to stand closest to Kate rather than the heir, the journalist furthered:

“At last year’s Trooping, you had William and his father shoulder by shoulder. This year, it was Kate who stood next to the king. I think that was very deliberate. The cameras caught them in a bit of an exchange, where they seem to be talking very warmly, very fondly. I’m hearing that always close relationship between Kate and her father-in-law is now even closer… They are both going through cancer treatment. They’re both going through good and bad days, and therefore they’re able to relate to one another and have some sort of understanding that probably they can’t have with any other members of the family.”

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Ultimately, Nicholl believes Kate was filled with “a great sense of pride” after the success of the appearance, musing:

“I think clearly a huge amount of effort went into her appearance behind the scenes, and it wasn’t just about how she looked. It was how she held herself. It was the pride that she took in being there, the way she shepherded her children into the carriage and up onto that balcony appearance,. It was obviously very, very important for her to be there on Saturday, and I think she went to great lengths to make sure that she could be, and to do it all with that wonderful smile on her face.”

Katie continued:

“I think she felt very much that she needed to be there. The children looked so happy and relaxed having their mother there… It was important, I think, for the Princess of Wales to be there to support her father-in-law, to be there for her family, and to be there for the nation to send out a really important message that she’s doing OK.”

While this appearance should “give us all a great sense of optimism,” the beloved royal is not out of the woods yet. Referencing the future queen’s powerful update on her chemotherapy treatment and thought process on attending more events, the royal correspondent cautioned:

“She was very honest in that message. She said there were good days, and there were bad days when she was tired… I think we have to assume that Trooping was a good day, but she is still going through cancer treatment. She’s spoken about this need to be able to go through this period privately for her own recovery, but also for her and her family to have that much-needed privacy.”

So, while Kate is expected to “attend a handful of engagements” throughout the summer, a lot will depend on how she’s feeling each day and if she gets the go-ahead from her doctors. Assuming the best, Nicholl suspected one upcoming outing could be to attend Wimbledon in July, expressing:

“I wonder if we might see her at Wimbledon. She spoke in that very personal message about wanting to be able to do the things that bring her joy. I think that’s being in nature, that’s being with her children, that’s taking part in everyday school life, but tennis, don’t forget, is something that the princess absolutely loves.”

And doing what the princess loves right now is vital, Katie concluded:

“She takes great pride in her position at the Lawn Tennis Association. Wimbledon is a firm fixture in her diary. Every year we’ve seen her attend with her family in the past. It wouldn’t surprise me at all if we see her pop up at Wimbledon, because I think that is something that will bring her joy, and a huge amount of her recovery depends on her being happy.”