hard to tell by the pic but could be mites..could be a heat rash of sorts...try washing with a vinegar/water wash and treat with iodine..if it does look like its going to heal up in a few days..then treat for mites..ivomec 3 times one week apart..1 cc per 40# Sub Q
We have these little tiny hard to see gnats that live in the damp ground near the water tub and such. They fly up and bite only the tender underside parts of the animals. They cause a rash like that as well. I use Bronco spray on the belly to keep them off.