Does my girlfriend still have feeling for her ex?

Hi,
I am starting to believe that my girlfriend might still have feelings for her ex even though she says she doesn't love him. She said that she started to like me back in November when we were both at a party together. I have always liked her but didn't want to say anything to her because has always been in a relationship. We were both at a club together about 2 months ago and we both said how we feel. She was still with her boyfriend at the time. She always used to talk to me about how her badly her boyfriend was treating her. For example, he always used to hang around with this other girl and make no effort with her. I honestly thought that she was using me for the same thing and didn't expect that she would actually break up with her boyfriend. About a week before she broke up with him, we spent a lot of time together. She said that she was trying to let herself hang around with other people seeing as though her previous boyfriend did the same to her. We both say how happy we are because of how things have turned out. However, she still keeps in touch with him every now and then and it makes me feel uncomfortable. We were talking today and she was saying that she had mixed feelings but she wouldn't say what her mixed feelings were for. I think she is hiding something from me. Our whole situation is weird because I met her through her ex boyfriend because I was friends with him. I had no intention of doing anything with her at the time because she was in a relationship. But it's always in the back of my mind that I am a rebound. She assured me that I wasn't on the day she broke up with her boyfriend and I had no choice but to believe her. Am I just being being paranoid about everything? What do you think?