Is my girlfriend having a relationship with another guy?

Recently my girlfriend has been spending an increasing amount of time with her friends with who meshe goes to hotels and drinks with. ONe of the members of that group is a guy that she has been talking about a lot recently. The other day she used facebook on my phone and I saw messages from him to her in which they carried on a lengthy conversation. She told me about this that night so i thought no worries especially since she told me that she wwasn'tgoing to text him. However at the end of their conversation he said he was going to bed and she said she was about to text him but goodnight anyway. Days later I see over her shoulder that she is texting this guy in much the same way that she used to text me: all smiley faces, responding immediately, flirtacious. Lately I have noticed that she is much less affectionate as well toward me and at one point she even asked me to leave her house early so she could go hang out with this particular group of friends. Whats worse is that when she talks about hanging out with me with other guys she referrs to me as her "friend". I need help to see if im just being paranoid or if there is something actually going on here that i should be worried about. I really love her and i wish that I wasnt stressing about this but I cant deny the evidence in front of me. Please help!