Okay you said you were her first. her first or her first time? obviously the first time there will be blood and most women say it hurts the first time.

Other than that was it dry sex? That could hurt, and the abrasions could start bleeding. Is she sick? Is she clean? (no offence here dude, just in the name of medical research) Is she on medication? Are you sick, clean or on medication?

How long were you porking her? 2 mins? 2 hours? Lots of reasonable explanations - if it happens again then theres a problem.