Why did she get mad when she found out I was using her?

No one likes to feel like you PRETENDED to like them, which is what you did here until you really started to have feelings. You need to explain that it was nothing personal to her & that you really DO want a relationship with her....If she doesnt forgive you... thats your own fault for being stupid & immature. You do NOT play with peoples feelings/emotions/head like that. Its just F'd up.