Why would sex suddenly hurt? (Abstinence advocates need not answer.)?

[Rockin' the Bunny Farm]

First of all, I'm not a virgin. My current boyfriend and I have been together for over a year, and sexually active for most of that time. Let's just say, he's rather well endowed, which took some getting used to but I did.
Last night, however, it actually hurt. It was like a shooting pain in my abdomen when he thrusted, which lingered for a few minutes after we stopped. Why?
Btw, I recently went to the gynecologist, so I'm fairly sire it's not an infection or anything.