Boob growth in early 20s???

So I’m 21 and I’ve always been really flat chested and up until 6 months ago I was AA cup (never bothered with wearing bras lol) in the past 6 months though I’ve gone up to a C cup. I weigh myself regularly and haven’t gained any weight. Around my period especially they swell so much and a C cup can be too small, I find it crazy how this is happening in my early 20s and am now having to get used to Bras ahah, has anyone else had this randomly in their early 20s??