Mate! It's normal, usually a thing called lack of sex OR she might be a reall hottie like my girl at the mo, she just has to touch me or stroke my arm and it's up and away. She just shakes her head, it's embarassing when we're out shopping, I just say that I have to have a breather and sit down. lol.
I think most guys suffer from it at sometime.
Hehe, you could always try a staple gun :P