Scenario: It happens. She grabbed your hair in that way, said your name the way you like—whatever it was, you were quicker on the trigger than normal.
Solution: Penises are famously finicky. No matter how many visions of baseball and grandmas (and grandmas playing baseball) you have floating through your head, you'll inevitably blow the game in the final moments every once in awhile. If she's cool (and not a one night stand) she should be understanding. Of course, she'll be more understanding if you make sure she climaxes, too.