My Girlfriend Is On Her Period?

I know she is, she has told me. But she is into dirtbikes, and about a week and a half ago she was upset because she was fighting with her friend and every1 was upset. We made up and she took a trip to illioni and had a bunch of motorcrossers hitting on her, asking for her number, calling her cute, .etc. She goes along with it and her and her friend always look and find the cute guys and like give them names and stuff. This frustrated me very bad, and she got mad about it. She finally made up with me. Then I told her something that I thought would make her happy, 2 girls wanted to come over and not only did i not want them over, but I told them STRAIGHT UP NO to make my girlfriend happy that she was my only girl I would ever need. She got pissed about this too and didn't text me. I was still.. nice about it. Called her babe. Constantly told her how I was on her side.. and I told her I loved her. Still she didn't text me and ignored my voicemail saying it was ok. She didn't text me till the next day. She finally got home from the trip from the motorcross track and has been talking to jensen about all these really cute guys she saw there and all this. And they were bragging about all the guys hitting on them. She now is not acting the same. I tell her I love her and she doesn't say the same. She acts different, and when I call her she sounds so much less enthusiastic. I told her I missed her on the phone today and it's been a while sense I got to see her face and she didn't say she missed me she just said "Ha.. and its only been 4 dayss" and stuff. I'm not too clingy, I've just been trying to back her up lately. This is seriously getting me upset, and I DON'T know what to go to. I don't know what to do. I'm just trying to help her and get her through this tough time. This is not herself :( Please help somebody. I love her, and can not let her go. She does not seem nearly interested as usual. Is this her period or what?