It clearly sucks and I'd hate it too, but it doesn't necessarily mean anything is going on either. I used to sit next to my mates girlfriend at work, he also worked with us but in a different department. She told me one morning she'd accidentally said my name while shagging my mate, and he was understandably pissed about it. She was mortified but thought she better warm me. I was amused and a little pleased but I knew better than to take the piss out my mate. I said nothing. Weeks later my mate raised it as a hypothetical in the pub and I just dismissed it saying it wouldn't mean anything, people spend so much time with colleagues and use their names so often it's easy to mess up. It's just really unfortunate if it's mid sex. He just grunted. Anyway, my point is that I was not shagging said girlfriend, nor would I have, and I had no obvious signs it was on her mind either. We just spent a lot of time together. Lastly, your girlfriends awkward reluctance to discuss it may be a product of her own disbelief, embarrassment, confusion, and utter horror at what she said.