Toilet paper. I just realized recently that Cottonelle really aggravates my skin. I have read that different people react negatively to various papers. I was just trying to think of some options for you that doesn't include the list of scary std's that everyone likes post. I suggest get mirror and see if it looks red. If it is red or even tiny bumps it could be a simple contact dermatitis. You might want to think if you have started young anything new in the shower or toilet routine. Laundry detergents and fabric softeners and even certain fabrics can be culprits too. I use All Free and Clear as my kids have very problematic skin. I suggest keep the area dry and try to let it breath as much as possible. At home maybe try no underwear with a long skirt or something to let the area breath more. I try to sleep in all cotton pants or boxers. I get this reaction when I wear cozy fleece sweats. Bummer. They are all polyester and that area just doesn't breath well and can get sweaty and have an uncomfortable reaction. Tight clothes rubbing can also irritate that skin. Anyway this is just a few simple things to consider. I hope it helps.
It is possible that this is from reaction to TP or any new soaps, shampoos or other body care products. Yeast could be a possibility too.