For a contact allergy like that, I'd think first of something you are using on the dog...shampoo, flea/tick treatments, etc... I think most dog allergies are causes by dander and/or saliva, which I think you'd find less of on the belly than on other parts. Do you use a spot on flea treatment? Those travel through the oil glands on the skin to cover the whole dog...belly would be where you'd get direct skin to skin contact.