Hurts a bit every time?

I hope I can explain this without going on too long. Basically, every time my boyfriend and I have sex, I always get a sharp pain when he first goes in, and it fades away after a few seconds. This happens even if we use a lot of lube, or if I've just came and am already very wet. It doesn't change no matter the position, or if he goes in slow.

I just sort of accepted this as my body getting used to it, but I mentioned it casually to my boyfriend the other day and he was concerned, so I guess now I'm second guessing. Mostly I just wanna know why. Anyone else have this problem? Would you say it's pretty normal? I can't tell if it's that I'm small, or my boyfriend is big, or I just tense up too much or what.