If she's cheating on me, why is she still denying it?

[James]

I work with this girl, and over time we got really close and intimate. We'd start hanging out everyday and we found out we have everything in common. She started acting like were dating by asking for hugs and kisses before I would leave. She even came over at 3am once and we had sex, and cuddled into most of the next day.
Here's where it gets interesting. I saw a facebook status and she tagged another guy in it. I didn't think anything of it at first, but I found out she's been staying at a friend's apartment the past few days, and he rooms there with them. He posted on twitter, you make me happy, I like you, the past few days have been great, and I wish you didnt have to leave. This is where I flipped shit. I texted her a lot of mean things because it seemed like she was playing me.
Her response caught me by surprise. She denied it completely, saying he wasn't talking about her, there was another girl that he had over, and that she was hurt that I didn't trust her. She said that she goes to her friends house cause she isn't good with her family right now, and it just givers her a place to escape to, and that everyone that rooms in the apartment are just friends.
I saw another shady post on Thanksgiving. He posted, "my boo is having a bad day, and its making me upset"... and she was in the er with strep throat. I called her out on it again. She sent me texts saying I really hurt her by what I was saying, and that I'm pushing her away with these accusations. She again said that he was talking about another girl.
Thoughts? I feel like a dick if she's being honest, but this situation is so shady. If she's cheating on me, why go to that extent and lie and make me seem like the bad guy?