My first guess would be a fungal infection. I'm no expert though.
You can read about it more here.
https://www.guinealynx.info/fungus.html
If it indeed a fungus, it's very treatable at home. You just need to get some of the ringworm/jock itch (1% clotrimazole cream) or Monistat-7 (micanzole nitrate 2% OTC human medicine) cream to treat it.
I'm currently battling a fungus on my two piggies, and purchased the 1% clotrimazole cream from Target. It was just on the shelves where they carry anti-itch creams. (Side note, they probably have two brand name ones that say ringworm for $12, a brand name that says jock itch for $8, and then a Target brand yellow boxed one that says jock itch for like 6 bucks. As long as it says clotrimazole 1%, you can get the cheaper yellow tube one. I asked the pharmacist, and they are all the same, just different labeling since most people don't know that ringworm and jock itch can be treated with the same stuff. She told me to get the Target brand one, since it has the same exact ingredients, and is cheaper.)
You apply it 2-3x a day (using a q-tip or cotton ball, as fungus are very contagious, even to humans), making sure to rub it in REALLY well.