While Prince William was shaking it off at night one of Taylor Swift’s Eras Tour in London over the weekend

Kate Middleton was noticeably missing from his side. But the Princess of Wales had an ultra-relatable reason for staying home.

According to a source at Entertainment Tonight, Middleton was on mom duty.

Taking care of her and William’s youngest child, Prince Louis, 6, while her husband celebrated his 42nd birthday at the concert with the couple’s two eldest kids, Prince George, 10 and Princess Charlotte, 9.

Swift is set to return to Wembley Stadium again in August, but don’t expect to see Middleton—who just made her public debut following her cancer diagnosis at last week’s Trooping the Colour—in the stands then, either. “Princess Kate is not expected to attend any future Taylor concerts,” the insider shared, adding that “her appearance at the King’s Trooping the Colour doesn’t mean she is returning to a full schedule of public engagements yet.”

Taylor Swift Took a Selfie with Prince William and His Kids at the Eras Tour

Apparently, her attendance at the birthday parade took a lot out of her, according to royal expert Katie Nicholl who observed the princess’s energy waning at one point during the event. “She was on her feet for a long period of time,” Nicholl told ET. “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 acknowledgment that she is not as strong, not as fit, as she was before she went into this treatment.”

Nicholl went on to explain, however, that there was “no pressure on her from anyone” to attend, adding: “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.”