Canker Sore Overview. Canker sores, also known as aphthous ulcers, are among the most common causes of painful sores in the mouth. 1  The classic lesion is a small (less than 1 centimeter) oval to round ulcer with a erythematous (red) border.2  They occur on the mucous membranes inside the mouth.anker Sore Overview. Canker sores, also known as aphthous ulcers, are among the most common causes of painful sores in the mouth. 1  The classic lesion is a small (less than 1 centimeter) oval to round ulcer with a erythematous (red) border.
Other Causes of Heavy Panting in Dogs. Heavy breathing or deep, intense panting can also be a symptom of eclampsia, also called milk fever. Eclampsia is a dangerous condition that affects nursing mothers; low blood calcium levels lead to an inability to stand or walk and tremors.
Dogs can have stridor, or croupy breathing, which has a raspy, high-pitched sound. Dogs can make this noise when they exercise. If the onset of this sound is sudden, however, this could be a sign that something is lodged in the dog's voice box. If a dog is wheezing, the trachea or bronchi could be having spasms or narrowing.