Are my mouth ulcers always canker sores or can they be something else?

Usually: If they occur frequently then most likely canker sores. If you have swelling and the sore is adjacent to a tooth that is sensitive or painful, it may be an abscess (infection) caused by either a tooth nerve problem needing root canal treatment or a gum problem needing periodontal treatment. Photo of canker sore

2 types: Typical mouth ulcers are caused by viruses and are generally 2 types: herpes or aphthous. They both last about 10 - 14 days. Aphthous ulcers are found on the loose, moving tissue in the mouth -- inside lips, under tongue, near cheek folds. Herpes are on firm tissue -- gums around teeth and roof of mouth. Herpes is very communicable and you can pass it along to other areas of the body (eyes, esp.).