Why does sex hurt sometimes ?

Jessica • Angel Mom of 6; Lailah Star 8/5/16 Expecting my 7th July 2018
So sometimes when y boyfriend and I have sex, it hurts me. It's only certain times though. There are times he can put it in all the way, and sometimes it hurts so bad I don't want to have sex. I don't understand. Wondering if I'm the o my one, or what this means ? Lol haven't done my research I guess