maybe you tore your frenulum? I heard that happens...
Happened to me once, I felt a bit of pain but kept going. Suddenly felt a sharp pain that made my whole body tense up. Pulled out and ran to the bathroom with my dick in hand. I was squirting blood on walls, floor, toilet, blood was everywhere. Didn't have sex for two weeks and took it slow when we finally got back into the groove of things.