Erythema nodosum (say "air-uh-THEE-muh noh-DOH-sum") is a condition in which red, painful bumps suddenly appear under the skin. They usually appear on the shins, but they may also appear on the forearms, thighs, and trunk.
This condition can be caused by a number of diseases and infections, or it can be caused by medicines. About half the time, there is no obvious cause.
It often starts with flu-like symptoms, such as fever and aches. Sometimes these symptoms come before the rash.
After a few days, the bumps turn bright red or purple and then fade to yellow or green, much like bruises. They usually fade completely in a few weeks.
Treatment is often not needed. But your doctor may do tests to try to find out what caused the problem.