I don't know if all the information is relavent but bear with me incase its important

I've been dating my girlfriend for almost 8 months now, and it's happen 2-3 times recently where all of a sudden I can't get turned on to do anything. I am 19 and she is 18. I'm not a virgin before this relationship and she was

We're currently living an hour and a half away from each other and we see each other about twice every 2 weeks (I would come home every 2 weeks and see her the friday and saturday)

Previously, say one and a half to 2 months ago, we weren't having sex but we would have these these amazing make out and oral sessions, but recently, it's happened twice that I couldn't get turned on to do anything.

The first time was when she visited me at my resident, so we'd be completely alone, we were teasing each other all day physically back at home but we wanted to wait till we were alone at my res. When we finally got there, we started again. She got scared that we were going to have sex (I wasn't even thinking that we would), we had a talk, and then after that I just couldn't get turned on after.

About 2 weeks ago, we were making out again, and she stopped me and told me she was ready for sex and pulled out a condom (we've talked it over a lot in the pass month and a half). And then we had a talk about if she was defininately sure, but I couldn't get turned on again after the talk, possible because I was nervous. We gave up but before we slept an hour later we did ended up having sex that night, and we had sex again the next morning

Then the next night came around, and we were alone in our basement, I guess we tried to have sex again but I couldn't get turned on to do it, I had this dreaded feeling, I couldn't explain it, we kept trying to get me turned on but it didn't work. I haven't seen her since then cause of school and the distance.

Now these 3 events didn't happen in a row, in between each one we've had at least 2 "sexual" sessions but I'm still worried what is wrong with me.

I've talked to some friends and so far I've got:

The common thing happening seems to be sex

It's possible because we only see each other for a weekend every 2 weeks, that we kind of force ourselves to be sexually active instead of actually wanting to be

Please give some input, thanks