Discovering a new lump or bump on your skin can be frightening. When located behind the ear, those bumps could be anything from an infection, such as mastoiditis, to an allergic reaction, such as dermatitis. Only your doctor can diagnose you properly and prescribe the appropriate treatment.
According to the government's Medline Plus website, ear infections can sometimes cause the mastoid bone---located behind your ear---to become infected. This condition is most common in children. When it occurs, the area behind the ear swells and results in a visible protrusion. This protrusion may be as small as a fingertip or as large as a plum. Additional symptoms include fever, hearing loss, ear redness and ear pain. Doctors treat this infection with repeated doses of antibiotics.
One particular type of dermatitis---known as seborrheic dermatitis---often occurs behind the ears. The Merck Manual website notes that seborrheic dermatitis causes scaly pimples to form; these pimples can be yellow or red in color. Scientists don't know exactly what causes dermatitis. They do know it rarely affects teenagers, is more likely to affect men, worsens with low temperatures and may be hereditary. To treat it, doctors may prescribe topical corticosteroids to loosen the scales and dissolve the bumps.
Your small bumps behind the ears might be a case of swollen lymph nodes. According to the government's Medline Plus website, your lymph nodes belong to your immune system. They're found throughout your body, including behind your ears, in your armpit and in your groin. When you develop infections such as tonsillitis, mono, a cold or a flu, your lymph nodes might swell up to the point where you can feel them. Your lymph nodes may remain swollen for days or weeks after the infection. In rare cases, cancers and tumors can also cause lymph node swelling.
Because the causes of rash-like skin bumps are so diverse, you should not attempt to treat them yourself. If, for example, you believe you have dermatitis and apply a steroid cream, it won't help if your bumps are caused by lymph node swelling or mastitis. While it's acceptable to watch and wait for a few days to note the changes and colors present in your bumps, you should call your doctor and describe your symptoms to her. She may be able to suggest a course of treatment over the phone or she may request you make an office visit for a firmer diagnosis.
In some cases, bumps behind your ears may indicate serious conditions such as lymphoma or leukemia. These cancers can cause swollen lymph nodes, which you may feel as bumps behind your ear before other symptoms emerge. If the bumps aren't painful but persist long after a routine infection such as a cough or cold, see your doctor. Medline Plus notes that painful bumps often indicate active lymph nodes fighting an infection; visible bumps without pain may be symptoms of a systemic condition such as Hodgkin's Disease that requires immediate treatment.