Most Helpful Girl
It's over with.
You can tell your girlfriend (although I don't think it's necessary).
Masturbation and nudity is a part of life.
Don't let that make things awkward.
Most Helpful Guy
There is nothing to do. I bet you do the same. If she acts weird around you, it means she is embarrassed. You should talk to her while alone and ask for her forgiveness, explain why you came in (why the hell did you come in btw?) and tell her it's ok and that you don't judge her. If she doesn't look embarrassed, it might be that she actually liked it. Just keep to yourself then.