Couldn't get an erection. That was it.

I, a virgin, was getting hot an heavy with my girlfriend this weekend. We both agreed we would hold off on sex. We were fooling around for several hours but would stop before we engaged in intercourse. This mean my penis would be hard for a long period of time and then soft when we stopped. This happened several times on Sunday. Then we finally decided to have sex later that day. The only problem was my penis couldn't get hard. I assumed it was because it had been erect for so long several times that my penis was fatigued. I was wondering if this was normal or if this was the real reason why I couldn't get an erection. I heard it may be performance anxiety but I had already been erect several times earlier and for long periods of time so I don't think that was it.