According to Med-Health.net, a cyst or lump behind the ear can be caused by swollen lymph nodes, sebaceous cysts or mastoiditis. Lumps behind the ear are common, and timely detection of these lumps can help avoid severe problems when identified and treated.
Med-Health.net states that a lump behind the ear is most commonly caused by swollen lymph nodes. Lymph nodes are part of the immune system and help fight infections in the body. Swollen lymph nodes usually aren't serious unless they're hard and immobile and grow rapidly. If a swollen lymph node appears as a bump behind the ear, it needs to be examined by a medical professional, especially if it doesn't resolve in a few days.
A sebaceous cyst also appears as a bump behind the ear, according to Med-Health.net. It contains a cheesy protein called keratin that gives off a foul odor. Sebaceous cysts occur due to either skin trauma or swollen hair follicles and typically don't require medical attention unless they become infected.
Med-Health.net notes that mastoiditis occurs in the mastoid bone, which is located directly behind the ear. Mastoiditis develops when an ear infections causes an infection of the mastoid bone and causes the area behind the ear to protrude out of the skin and become swollen. Ear redness and pain, fever and loss of hearing accompany the bump behind the ear that forms due to mastoiditis.