The problem with three-somes is someone always feels left out. If this is true for your girlfriend then I can understand that she feels as if she's been cheated on.

It's just a suggestion on my part, be gentle with her and reassure her that you are the only one for her. Expect some finger pointing but don't point back. Just remember you love her and be patient. The outcome may or may not be what you hope for but just do your part to honor her feelings.