Why is she doing this to me?

To be very honest with you, your friend Jill is a being a B****. That being said, she obviously has more confidence than you and that can and often does win a guy over- even more than looks, height and money. If Bob is really into her then youve missed your chance, for now, but if not then you have just as much of a right to try to win him over as she does. Idk if you know this but guys will play both sides and try to hook up with everyone if possible so dont fight over him, but you should tell him how you feel and ask him to be honest and straight forward with you. If he likes Jill instead of you then just try to get over it for now but if he likes you then you should pursue it and she'll have to get over it. This sounds like it could get messy though so put your big girl panties on and do what you have to do but be really careful.By the way, if youre just a kid, youll like someone else in no time so dont lose sleep over it or anything, youll be fine either way.