Painful Sex...?

I haven't been sexually active for very long, maybe about two months, but yesterday my boyfriend and I were ~getting it on~ and he went to thrust all of him inside me (keep in mind, he's pretty well-endowed) and I felt pain. A lot of pain. It was similar to my first time having sex, but this time it hurt much more. 
We continued doing whatever, but everytime he would go balls-deep, it would hurt. 
It was weird because I hadn't felt pain during sex since my first or second time and it certainly had never hurt that badly. 
Help???