MY boobs go from a small b to a small c during my 32 day cycle its normal
BTW, mine do swell a bit with hormonal fluctuations. If you are nearing your period, this could happen.
Well, one of those things, I guess. Perhaps too your bra shrunk in the wash. Anyway, no explanation. I am large chested. I find it a bit of a burden myself. Oh well, that's life. good luck