She might be coming out of another relationship that she has not let go of yet. she days 'this is going too fast' because she has not gotten over her feelings for someone else. if you really like her, give her the space. if it is just for sex, find someone else. she is probably hurt from the last one, and she is scared.
on the other hand, she might still be involved with him, and you are the one who can break them apart.
regardless, good luck.