Is it normal that i like to masturbate while watching my girlfriend sleep?

My girlfriend and I are very sexual. We've done just about everything that's "Normal" besides actual sexual intercourse. (we're waiting) We haven't moved in together yet, although we are very close and are discussing our future together. Most of the time when she comes over, she falls asleep at some point, and (Unless we've just done something sexual) I usually have an uncontrollable urge to masturbate. (Especially if I'm lying next to her) She knows that I masturbate, so the actual act would not be a surprise to her, (Sometimes I end phone conversations by telling that I'm about to go "Jack off") but I'm afraid of what she would do if she caught me. I'm not sure what she would do, considering the fact that on our first date, (When we were 15) she pointed out my erection and said she was turned on. I'm not sure if I should tell her, if I should just stop altogether, or keep doing it...