Aphthous ulcers, or canker sores, are a common cause of pain for many people who suffer from oral health issues. From the insides of your cheeks and lips to the bottom of your gums and even under your tongue, these tiny but excruciating ulcers may develop anywhere in your mouth. Differentiating canker sores from fever blisters—which often appear on the outside of the lips or at the corners of the mouth—is crucial.