need medical advice

So a couple of weeks ago my waist line was a little bit itchy however as it itched one spot I felt my fingers starting to get wet so I lifted them into the light to find blood on them. I was a bit stumbled on what it could be so it went to the bathroom and stood in front of my long mirror to find a cut (or something like that) I was freaking out because I had no idea what it is or how it got there. I put some antiseptic cream on it and did that for a few days then completely forgot about it until now as it started to inch again. However it now seems to have some sort of white gunk in it and I'm freaking out more trying to figure out what it could be. It's a couple centimetres in diameter but it seems deep. I don't know what to do. Also I'm only 17 and I'm not the sort to talk to her parents about these things.