What movie (or book?) is this?

This is all I know. It'd be toward the end, Okay:

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A girl (let's call her A) sees a guy (I think it was her boyfriend)
about to kiss another girl (B), so she runs away and goes to the
bathroom stall and cries.

B MIGHT HAVE come into the bathroom after A, but then I think she
didn't say anything when she realized A was crying.

At some wedding, the guy doesn't wanna go in because A is there, but
someone talks to him and convinces him to, so he goes in and sees A
and she smiles at him.
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I'm sorry that's the best I've got. MONTHS ago I was in the doctor's
office and I heard some lady describing this and I had a notebook on
me so I just wrote down what I heard/remembered her saying. XD LOL.
But anyway, I've had the little note for so long now that I decided to
finally ask on here to try and figure out what this movie (or book?)
is. So does anyone know? Thanks in advance. :)
What the... how did I ask this on Y!A CANADA?! I'm from the USA! LOL.