They are called nipple's that means your dog is a girl
My Dog Has Little Red Puss Filled Pimples All On Her Belly Side, What Could This Be? They Do Not Seem To Bother Her At All.
Could be Acne or from where she rests. Could be Bacterial and should be checked by a Vet.
I wouldn't bet on acne as it doesn't usually make its first appearance in that area or in that manner among other things. It probably needs treatment (staff/bacterial infection or contact allergy most likely both of which will need treatment) and you may need to change your food as most dogs are severely allergic to the horrible things in most commercial dog foods and many of the ones that people think are the best are some of the worst. Skin problems and infections, ear infections, paw licking and chewing, excessive tear staining and many other things can all be signs that you should change your food. They look better, feel better, less shedding, fewer stools, less vet bills due to allergy related problems and they live longer. It is usually a major factor in your food. Avoid foods that are mostly chemicals (like ethoxyquin), preservatives, fillers (which just make your dog feel full with none of the nutrition so they eat more and you know what that means, and corn for starters. See your vet for treatment and do a very slow switch to a more natural holistic food and see what happens. You usually see a huge difference by the end of a medium to large bag. Here are some resources. Hope your pup feels better soon.