How would I gEt rid of, what I presume is a rash?

Well, it started like 15 hours ago, when I was at work, it wasn't really itchy but I felt something on the right side of my left hip, I don't go to the bathroom much, and I just thought it was my underwear or something, when I came home I forgot all about it, but an hour ago I was going to go take a walk but then I fElt it again, I went in my room, took my pants off and looked at it, it looks like a rash, it feels like what I think a rash would feel like as well, should I worry about this? What would help it go away? I'm just 13, not allergic to anything but, I have been wearing the same underwear lately, because I wake up late and don't have the time to look for some, so should I like just get a new pair and it Will go away? Please help me, T-T