Hi Andrew,
Communication is something which can easily solve many problems and doubts. No one here, including me, can give you an exact answer to what's going in her( your girlfriend's ) mind. Instead of clearing your doubts here on the forum and get muddled up by various responses, its way better to sit and talk to her. Whatever doubts you have in your mind, the answers to all them rests with her. So, I would advice you to go to her, tell her everything that you are thinking, all your doubts about everything ( including that guy ) and solve your problem easily.
I believe, you guys are close enough to have involved yourself in physical relationship. There should be clear communication between both of you. That will clear the picture to much an extend.
All The Best !!!
Take Care !!!