Is she cheating on me??

I'm starting to think that my girlfriend of 5 months has been/is cheating on me. Heres some things I've noticed.

-Early in our relationship, she admitted with having sex with a girl in her neighborhood. Since it was with a girl, and not a guy, I did'nt care much about it.

-A month ago we went over to a friend of hers house, there her Ex Boyfriend was living. We visited for awhile, until I had to leave, so I did. The next day, she told me she'll probably go over there again, "to help clean their carpet". She could tell I was'nt happy about this, and changed her mind.

-She was talking to her ex boyfriends WAY too much for me, she'd call them, chat with them at the local cafe, etc. After I told her I did'nt approve of this, her nosey best friend stepped in and said I should'nt be soo jealous.

-For Christmas, I spent around $150 on things for her. She got me a used puzzle, and a drawing that looked like she traced(she made the same thing for all her friends). I don't care if she buys anything, she could've MADE me something out of cardboard for all I care.

-I recently went to a party, when I got back, she told me. "You can sleep with anyone you want, I don't care as long as you don't give me any STDs." This REALLY got to me, she better not expect me to be soo lenient with her cheating!

-Early in the relationship, she informed me that she had been molested when she was a child, and thats why she hates having sex. I accepted this, and decided to be a bit more understanding when she did'nt want to have intercorse. She also said, I was the first guy she willingly had sex with, and all the others had basically forced her.
She COMPLETELY contradicted herself today, when we were playing "the weirdest place you did it at", and she said, "In a classroom, I was REALLY in the mood."

And finally, her mom and sister are both known and confirmed cheaters. I saw them cheating infront of me, numerous times. And I fear she'll think it's ok, because they do it.

Am I being paranoid? Please help! Tweet