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

Well think about it hmm if you were in her shoes wouldn't you be mad? She was developing feelings for you and you hurt her. If you genuinly want to get her back Go to the store by a sweet hallmark card maybe a teddy bear and some flowers. Write in her card your sorry you hurt her and that she has a right to be mad tell her you genuinly care and have feelings for her and you want her back. Tell her you were a stupid jerk and messed up ask her to forgive you and give you another chance. Id she asked u about it say u never meant to go threw with the stupid bet and u developed real true feelings for her the flowers maybe some chocolate and a stuffed animal will help as well also don't make the same mistake again