welcome to the club
just keep your mouth shut and go with the flow. My ex gf (of 7 yrs) was quite experimental also. ..it didnt start off that way, but over time and taking "baby steps" about involving others into our relationship, things worked out just fine. She was a stripper and always bringing women home for us to play with, but nothing was said to my friends about what happened behind closed doors....people knew, it just wasnt talked about.
Also, theres nothing wrong with your girlfriend bro, she may be curious, or she might just be going through a phase in her life where she wants to experiment with her sexuality. Its good that you communicate with her VERY openly about situations like that, things can get turned around easily if assumptions are made. Just remember, keep your mouth shut, and go with the flow! good luck brother.