G.♣.
Google images of "Harry Potter the Fat Lady." I found images of her in a white dress and images in a pink dress - which is weird, because I would have thought the people in the portraits didn't change clothes.
She is the character who was singing and trying to break the glass. That scene took place at the beginning of "Harry Potter and the Prisoner of Azkaban" - the night the students returned to Hogwarts.
ETA - Same lady (well, character). The pink dress is from "Harry Potter and the Sorcerer's Stone." The white dress is from "Harry Potter and the Prisoner of Azkaban." The character of "The Fat Lady" was played by a different actress. The character of "The Fat Lady" appears in every book but only in those two movies.
It does seem like a major continuity error. The portraits should look pretty much the same in both movies, even with different actresses, but that doesn't seem to be the case. The two movies did have different directors, so that might have something to do with it. But it really doesn't matter why, the fact remains, both of those ladies really are "The Fat Lady."