Having rashes under armpits is not only uncomfortable but also embarrassing and unsightly. How you are going to treat underarm rash will depend on what caused it. darkened, thickened skin. When is a bump and/or lump on your pet something to worry about? In most of the cases, the rash is often itchy, irritating and very painful at times. Do not use the remedy without diluting it as it could be too harsh on the skin. 5. Yeast infections in dogs are fungal infections that can affect the urinary tract, mouth, or skin, especially the skin on the ears, paws, armpits, and around skin folds or wrinkles. A heat rash usually makes your pooch’s skin itchy and it might, in turn, bite or chew and lick the red rash on dog’s belly.