Why would she act this way?

Okay. This girl is insane. She is too immature for you. Your best bet, if you really want this is to lay the cards on the table and stop allowing her to toy with you, which is JUST what shes doing. I you want her to be your girlfriend (I pray you dont, from what you write she sounds like a 5 year old), ask her. If she says no, its no. No touching, kissing, etc. Just friends. Then see her reaction. I think she knows you're not going anywhere and is taking advantage of the situation to make herself feel superior. Women can be cruel....and usually are the cruelest to the least deserving men. Good luck!