Not understanding?

Holly • Hi! I`m 21 years young! School full time, while I work part time. I have the most amazing boyfriend in the world, we are like each others best friends and couldn`t be any more happier! ♡
OK so, my breast are becoming bigger by the week. Not understanding why? I even had to go up a bra size. No I'm not in puberity I'm 21 years old, also my nipples have been extremely sore. To the point where I can't even touch them. Any advice? Yes I am sexually active. This is odd to me.