It's perfectly normal.

Foreskin is like that but it stretches over time (just takes a little bit of sexual activities, by a little I don't mean once or twice) You can also stretch it yourself. I'd recommend you doing it the shower. Your penis head probably pretty sensitive too. Try to ignore it and do the stretching, or just explain to your girlfriend that it's fine, it's going to stretch and going to retract without any problems in the future.