It could be an allergic reaction of some kind. You may also want to check your pet for fleas or other pests. If the rash is itching her, you can give her 1/2 a Benadryl caplet per 10lbs of body weight, but no more than 2 maximum. Meaning if your pup weighs over 40lbs, you give just two. As with any problem concerning your pet, your veterinarian should be consulted for the best care.
The most common reason for the rashes on your dogs belly is a bacterial skin infection. It could also be mosquito or blackfly bites. These always need to be treated with antibiotics prescribed by your vet.