Sometimes it is easy to mistake a new puppy's pink belly for a rash. If the mother dog allows it try laying down a cooler cloth made of silk, because it will cool off the skin of the belly. If condition persists contact your licensed veterinarian.
You should contact your Vet to rule out allergies and such, but if it simply looks like dry skin, you could try putting a little olive oil on it, it is very moisturizing and won't hurt your pet if licked off.
Aloe is very good for humans as well as puppies and dogs. If the rash is more severe however. You could even used some hydro cortisone creme. If this doesn't solve the problem contact a vet immediately.
