My bf's dick hurts me during deep penetration

I am not sure if it's because of his big size or if something is wrong with me... during deep penetration, I feel his dick touching somethig inside me that hurts and feel uncomfortable and I can't keep up with sex when he hits it deeply. The strange thing is that I never before had this problem until the last 2 months (and we've been dating for 3 years) nevertheless I have to say we started to date at 17 and his dick seems fo have grown a few cm... could this be bcus his dick is too big for me? Or could it be something inside me that is swollen? Or a cyst?

Update: thank you guys for the comments, I've done some research about it now...