Is it still a rebound relationship?

My girlfriend of almost 9 months broke up with me a little over a month ago. We were very much in love and she always said I was the best thing to happen to her. But then one of her friends told her he loved her and she said she started having feelings for him. She said she didn't love him but she just had this "what if" feeling. She broke up with me because the feelings didn't go away even though she said she didn't love me any less. She's known this guy for a while before she met me and they really liked each other when they first met. They never dated though. She said that they slept together once but after that he said they shouldn't be more than friends. She moved on and eventually met me. She told me about their past but it never really bothered me because it was so long ago and she told me there were no longer any feelings between them. Now it's only been a month and she and this guy are already openly dating. I talked to her after I found out but didn't tell her I knew. I hoped that she would fess up to it but she didn't. It's really been bothering me since I found out and I don't know how to deal with it. It just seems so soon for her to be moving on. I've heard that rebound relationships rarely work but I don't know if this really counts as a rebound relationship since she seems to have left me for this guy.