Redness of the skin or gums in pets is a common condition that can be indicative of a vast array of health issues. The dog or cat has a discoloration of a mass. The redness is caused by blood. For example, when dogs or cats develop redness of the skin, it may be caused by various skin infections, contact dermatitis, flea allergies, mange or food allergies. Redness of the gums in cats and dogs may indicate such things as gingivitis, periodontitis, toxicity or heat stroke.