If she says it hurts at any point or is walking like a cowboy on the way to a stand-off after you're finished, don't pat yourself on the back. It isn't because you are hung like a horse (you probably aren't, that's just simple statistics).
It might be because you're an insensitive lover. Sex is about rhythm. Don't go thrusting away like a piston and assume she's enjoying herself. Get in tune with your lady. Does she want power humping, or would she prefer something a little slower and sweeter? Chances are, it's the latter.