This happens from time to time. It is generally a mental issue. If it happens once, most men tend to dwell on it and this in turn makes you unable to achive one the next time. Relax and go with the mood. Try and put it out of your mind while you are making love. If you are truely enjoying your sexual experience with your girlfriend that should be the last thing on your mind.
You should watch less porn and you will be ok. Do not masturbate for 2-3 days and go to your girl;)