sounds like it could be a staph
pyoderma infection of the skin. you can google this and also look at the site " dogandhorsecare.com/staph
-infection" it will show a photo of it. Or just google "photo's of canine staph
or skin infections on belly area".
I had to take one of my boys to a doggie dermatologist, he has severe allergies and also had some yeast and staph
infections on his skin, especially belly/underarm/thigh areas.