Girlfriend problems think shes cheating?

Okay so i have been going out with this girl for two weeks now we like each other a lot. But just the other day she started using facebook shes constantly on it every day shes always tired and every time i ask her its because oh i stayed up on facebook until like 12 last night. Shes starting not to come to her first block cl i know this isn't my problem but i know what shes doing on facebook she is talking to this guy i think she may be cheating on me with him im not sure. she logged onto my phone on facebook one day and she forgot to log out so i didn't even notice until i got a facebook message from this kid to her he said okay that sounds sweet but her message before that was oh my parents dont care they let my boyfriend pretty much live here during the summer. I have stayed the night at her house before so I took it as if she invited him to stay the night... then she posted this as her facebook status "i need a guy best friend lol i think i know who he is...... blond hair blue eyes skinny, kinda tall well taller than me who isn't taller than me haha... back in touch with him and everything is going good so far.... :)" she mad me extremely mad the other day when we were talking on facebook and it was taking her about 5 Min's to respond to my IMs and she said she was in a bad mood but yet she is all happy with this other kid commenting on his stuff ugg i dont know if im crazy or what. Today at school she didn't really seem like she was interested in me I can tell because she constantly wants to hold my hand but today she didn't really she also kisses me at random times but today she didn't even kiss me good bye.