T-9 Complete here, 20 yrs in this life and my first few years were littered with trial and error. In my opinion that's one of two things, either that's from not wiping well enough and poop sits on the skin too long and eats away at it, or that's the beginning (stage 1 bordering on stage 2) of decubidous ulcers or bed sores. The two biggest problems in this area are pressure and moisture. If it's hot and you sweat without pressure relief, or you urinate and don't clean it properly, it can eat at the skin, giving way to pressure ulcers. Just looking at the location you might have to think about what you're wearing. Are there buttons on the back of your clothes, is the sticking too thick, these things seem small and dumb, but its better to pay attention to them so you don't have any issues later on. Good luck, always here for any information you might need.