I wouldn't worry too much about it. 7 inches is a very respectable size, no matter how small it gets when flaccid. It's also fairly normal for guys to lose their erection (unless they're turned on mentally as well as physically) when they stop masturbating, and for most guys it's common
for the intensity of orgasm to make continuing masturbation a bit of a... well, sensory overload, for want of a better phrase.
Just enjoy yourself with yur girlfriend... In today's media there is so much emphasis put on girls being pressured into sex, and not nearly enough attention on how guys are made to feel that they should be able to perform at any age, whenever a woman wants. It's your body - wait until you're ready! It'll be worth the wait in the end.