Is she cheating?

My girlfriend just told me about this guy named Terry that she's been texting. I can't get any information about this guy out of her. Her excuse for it is that I don't bring her around my friends. We've been dating for 4 1/2 years now and when we first started dating she told me she didn't want to. I did take her to my best friends wedding and she's met all my closest friends. I asked her a few simple questions about Terry like how long have ya'll known each other and why won't you tell me anything about him. She goes berserk with anger when I ask. She kicked me out one night and said she wanted to hang out with him alone. I just found out that he visits her in the morning before he goes to work and I get out of bed. She just told me that she can't spend time with me sunday because she's going to dinner with him late and won't give up the details. At first it was just that she was going out to dinner late. I asked her what was up and she said it's just dinner. I than asked if it was someone else then she said Terry. So far I know that they couldn't have slept together due to her roommate being there. I am horrified to think that after dinner she will go back to his place. Our sex life is becoming stagnant maybe 2 times a week. Needless to say not as good for her as it once was. Don't get me wrong she gets off just mainly from me going down on her. With us not being intimate as frequently I don't last as long. Discussed it with her and no change. Now I'm building up with anxiety over Terry. Her birthday was last week and Terry has money so he bought her the same stuff as me ironicly just better quality. She swears he's just a friend but I've heard that in 2 other relationships and caught one. She also acts loving and we hold hands and she caresses my hand and arm while watching tv. She tells me she loves me but not as much as she used to. I try to be romantic and rub her feet most days I see here and have candlit sensual massages when ever I can get her alone. The biggest differance is our senses of humor are only partialy in sync. Her biggest pet peave against me I've noticed is that I am a large man and both our apartments are small so I'm in the way a lot. I know I can be jealous but in this case am I letting my past influence me or do ya'll agree that she's probably cheating?