There are numerous reasons why a lump could develop behind your ear. The most common causes are also the most benign, but some more serious ones also exist:

• Acne: Pimples in the ear can develop wherever a hair follicle or pore becomes clogged. The skin behind your ears is no exception. While pimples are less likely to develop there due to the area’s relative isolation, it can happen.

• Lymphadenopathy: This is a fancy way of saying a swollen lymph node. They are ovular, vaguely kidney-shaped organs that are distributed throughout the body. Lymph nodes also produce lymph fluid in response to infection. There is a lymph node in the neck that can sometimes appear as a bump behind the ear if it begins to swell. In and of itself, a swollen lymph node is not cause for concern. It happens as part of the body’s response to many different conditions and can sometimes persist for a while, even after the infection itself clears up.

• Infection: Any infection that causes swelling in the throat can cause a bump to appear behind the ears. Mononucleosis, for instance, commonly occurs with throat swelling and the above-mentioned lymphadenopathy. Obviously, the severity of your infection will affect how much of a concern the ear lump is.

• Lipoma: A lipoma is a fatty lump that forms between the layers of your skin. These can appear anywhere on the body and are almost always harmless. Depending on their size and the layers they form between, a lipoma is not always noticeable. This generally clears up on its own.
• Cysts: The main form of cyst that can appear behind the ear is an epidermoid cyst. Your skin (epidermis) is topped by a thin layer of cells that your body regularly sheds. Sometimes these cells move deeper into the skin and begin replicating when they are supposed to fall off (i.e. due to a damaged follicle or oil gland). These cells form the walls of the cyst and begin secreting keratin.
• Sebaceous cysts: Epidermoid cysts are sometimes called sebaceous cysts, although this term is misleading. A true sebaceous cyst originates from oil-producing sebaceous glands and is filled with sebum, not keratin. Having said that, true sebaceous cysts—while rare—can also form behind or on the ear.
• Abscess: Abscesses and cysts are similar in the sense that they are enclosed “capsules” filled with fluid. The main difference is that an abscess is filled with pus. An abscess can develop around foreign bodies, infections, or form out of existing cysts.
• Mastoiditis: The mastoid bone runs directly behind the ear. Ear infections can sometimes spread to the mastoid bone and result in swelling or lumps behind the ear.
• Tumor: A growth from the tissue of the salivary gland, mastoid bone, or skin behind your ear could be a tumor. These result when cells multiply uncontrollably instead of self-destructing at the end of their life cycle. While only a malignant tumor is cancer, a benign one can still be problematic. Depending on its size and placement, a benign tumor could affect hearing quality or cause dizziness.

Please consult a good ent doctor in your proximity to get yourself examined and start the treatment.