I need help ASAP please!

What my situation is, is that I am 21 years old with a little one and I have really no one to talk to about it. One day while at work I found out that my girlfriend had been uniting this guy over and I she didn't tell me about it. Later I confronted her in a very calm matter about it and she said nothing happened and she was going to let me know about it....that was three months ago and I have been finding out a little more every so often and now she said he almost pulled out his soldier infront of her and he gave her a kiss on lips but she said no. Later she deleted him off Facebook and blocked him and I have an iPhone(im posting it from my phone so she can't really see what I'm doing) I log on to her Facebook and check messages and her email. She has a history of cheating but here's the kicker, we have a child together. I don't know how to talk to her about if she really did cheat on me or what's going on. I have a gut wrenching feeling like something happened but I can't run on assumptions ( make a ass outta you and me ). Is there a way I can catch her or get her to tell me because trying to balance all this at 21 is really slowin me down. I don't want my kid to grow up and call me dad when there's another man coming into this house and she treats him like how she treats me.

1 sex life has all my dissapeared
2 she's gotten more interested in going to the bar
3 anything that's happened she always let's me know in pieces
4 she always pulls ( I didnt do anything it was all him ) card

Sincerely, someone who needs to find out whats going on before I get blown over by this potential storm that I don't see coming