What should I do? My girlfriend is texting a guy work friend constantly...

I just posted a question... but I also have another that's been bothering me alot. My girlfriend just got a new job and she has become friends with the only male co-worker who is also dating someone. She talks about him alot and is always texting him even when we are out and stuff together. She tells me "it's so innocent, if only you really knew" and that "we're only friends, i get along with guys better." So I let it go for awhile but it's really been bothering me. For some reason, I just don't trust this guy. She has had guy friends before who were great, I was even friends with them... but I dunno about this one. He does have a girlfriend, but he also goes to the strip clubs all the time and talks about the girls that hit on him. So it obviously makes me uneasy. This was all fine but then today, I get a text saying shes going out with him for lunch after her shift. She tells me to trust her, and I do.. but I can't help but think the worst. They work together almost every shift, and I have told her many times how it bothers me, but she says i's okay. Whenever I really get into it, she brings up a very unattractive girl I had worked with who had a crush on me and was always taking to me on facebook... but the difference is I barely replied because obviously I wasn't interested in anyway else. Please help, am I in the wrong for worrying and being bothered? is it normal and innocent or should I be more serious?