Nervous and losing erection

Alright, here it goes. I feel extremely embarrassed and don't even know what to think. I'm 18 years, and I am just a normal guy. I got my first girlfriend (yeah, my first since middle school or so, but I won't count them) about a month or two ago, and things are going great.

Usually, when we make out, that alone will give me an erection. Well tonight, she came over to my apartment and had voiced that she wanted to give me head. This excited me, and for the past few days I was pretty happy to see her again. Well, tonight rolled around, we get to the apartment and she's going to have to be home at 12. My apartment is 30 minutes from her house, so we figured we should leave at about 11:30.

We're making out and cuddling and stuff, and I have an erection for a long time. As it grows closer to 11, though, she keeps looking at the clock, and I know she is waiting for 11, to give us 30 minutes before we had to leave. Well as it gets closer to 11, my boner disappears and nothing that we do will bring it back.

She wasn't too bummed out about it, she said we'll just try again soon, but what in the hell? I am so unbelievably I don't even know... upset right now. She thinks it's her and I know it's because I'm just nervous or something. What the f?