I need some help about this girl?

I know this girl likes me we really get along. We hung out a couple days with a group this week and I got her number. I had a great time and we talked alot without the group. I even talked to this 21 year old for her even though I'm only 15 and told him to back off or I'd whip his *** if he kept messing with her. It seemed like she appreciated that. So I texted her yesterday and she replied at first but not when I texted her later that day. I texted her today and she hasn't replied. Has she lost interest? Am I coming off as desperate? If she doesn't reply should I wait a few days to contact her?