My pigs get these from time to time, esspecially Wendall. The first time they both got them I booked them in for the vet, but the appoinment was in a few days, by the time it came to taking them in the sores had gone away. Wendall gets them maybe once a month or so, but they only last a few days and go away. I'm not sure what it is that is causing them, I need to keep a better record of foods.
I would say to just leave it, if it is getting better and there is no blood or infection.