What else does she have going on in her life? Is she stressed out? Problems at work or with family or friends? Financial problems? Depression? Problems with school?
She could just want out of the relationship and by cutting you off sexually she is hoping you will walk. It's a passive-aggressive way to end things, but I see it as a possibility. She sounds young and maybe she is wanting to date other guys.
She went from 100 to 0 for some reason or reasons. Trying dialoguing with her and see what happens. If she isn't opening up and the sex situation remains the same then you should probably reevaluate this relationship.
This isn't uncommon. Lots of women go through phases where they completely lose interest in sex. I think it's a brain thing, or a hormone thing, and isn't related as you might think to prior bad relationships or current bad relationships.
Is she on a hormonal form of birth control? That can zap libido to zero.
I totally agree with SpecialMom that COMMUNICATION is the key.
You're going to have to talk to her about it. Don't be a jerk about it, just have a normal, adult conversation where you talk about the relationship and what's going well, what could use some work, and what are your plans for the future. Good communication is the foundation of any healthy relationship. Without it, all relationships would fall apart.
Hi there and welcome. Well, ask her why she NOW doesn't want to have sex. Did you convince her to do it in the first place? Maybe she feels guilty? Maybe she doesn't love it as much as she thought she would? Maybe she is afraid of getting pregnant? You need to try to communicate with her. And respect her decision.
It can also be a sign that the relationship may be winding down. good luck