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.
Congestive heart failure can cause heavy dog breathing because it means your dog's cells aren't getting enough oxygen. His respiratory system tries to compensate for this by working overtime, leading to labored dog breath. If your dog is suffering from heart failure, he may breathe easier when sitting upright.