Hi! The armpits do seem to be one of the places which matt up quite quickly, particularly if the dogs wear a harness. Could the harness be rubbing and causing soreness? If the matts are large, they might be rubbing and causing skin irritation, you could try cutting or clipping the hair in the arm pit to prevent it tangling up.
If the area looks infected though, - wet, red or smelly or if your dog is visibly uncomfortable, then I'd get the vet to have a look, it could be an allergy or some sort of parasites. Good luck!