The 36-year-old royal, who had just welcomed her third child Prince Louis, was believed to be feeling "quite emotional", a palace insider dished.
Kate's rumoured tears come amid claims the royal sisters-in-law are "not getting along", especially after news broke that Prince Harry and Meghan had decided to move out of Kensington Palace and into Frogmore Cottage.
Camilla Tominey, of The Telegraph, reported that two sources told the newspaper that Kate was "in tears" following her three-year-old daughter's bridesmaid dress fitting.
An insider told the publication: "Kate had only just given birth to Prince Louis and was feeling quite emotional".
Kate welcomed Louis on April 23, less than a month before Meghan and Harry walked down the aisle on May 19.
So it wouldn't be surprising if she was feeling a little overwhelmed, especially as eldest son Prince George was also acting as page boy.