Ulcers start when the top layer (surface) of skin or mucous membrane is hurt. This top layer dies. When it dies, the skin or mucous membrane and opens. This leaves an open sore called an ulcer.
Ulcers can be from infection with germs. Ulcers can be from pressure on a part of the skin. Ulcers can be from substances that will burn skin or mucous membranes. When acid makes some stomach ulcers this is from burning by the acid. Ulcers can be from cancer. Ulcers can be from diseases of blood vessels. If not enough blood goes to a body part, it can get an ulcer. Stress can also sometimes cause ulcers. They are also noted to hurt a lot.