Happened to me for two weeks during the 6 weeks I had stopped hrt before SRS surgery. I think it was the sudden drop (change) in hormone levels that triggered it.
During that time (about a month) my boobs went from an A-cup to a B-cup, yay!
Then after that they went back to an A-cup, and stayed that way ever since, boo!
I must admit that during that time, I did feel pretty amazing.
That was over 13 years ago now though.