Punk, my cold cold heart goes out to you... you are making yourself crazy.. you both have to relax... you're gonna get old before your time...
Your girlfriend is young.. anything can cause a menstrual cycle to go screwy...
Go to a local
Planned Parenthood and request that a test be done, while your there, maybe some birth control education would be in order. (and that wasn't said to be insulting, but you guys seriously need some education about the female body and reproduction.
Sex is a wonderful thing, when you are ready for it - this angst that you go thru maybe is just a sign that you guys just arent' ready... and that's not a bad thing...