Is she tring to make me jealous or telling me to back off?

She probably gave herself the hickey. It's not hard to do. I'd say she probably is trying very awkwardly and quite hilariously to come on to you. Now I suppose it's up to you to decide if you want to be ociated with a girl who gives herself hickeys. How old are you crazy kids anyway? This sounds like middle school, so keep in mind that the coming events could haunt you through high school. Not trying to scare you, just being honest about this very awkward time.