There are a number of reasons why lymph nodes swell behind ears. The lymphatic system consists of nodes and ducts that are spread throughout the body. They bring the lymph (which is the tissue fluid surrounding the cells, the fluid from intestines and some red blood cells) into the veins. Lymph also contains a small concentration of infectious substances. The lymph nodes are clusters of cells that are surrounded by a capsule. The ducts go in and out of the nodes and the cells in them produce antibodies that act as the cleaner of the cells in the body. The lymph nodes are where foreign substances and infectious agents interact with the immune system's cells. The swelling is normally caused by this interaction. Infection is a possibility, this increases the number of white blood cells, and they multiply in response to stimulation with a foreign substance. Another reason is a virus, which is when the immune system reacts to an infection within the body. This infection could be a common one such as a cold, or something more serious, like HIV. They can swell because of inflammatory cells during infection in the region of the lymph node. Cancer of the blood is also a possibility with the uncontrolled, malignant multiplication of lymphocytes, such as lymphoma. Lymphoma is a cancer in the lymphatic cells in the immune system. Swelling of a lymph node is normally nothing to worry about and the swelling will normally calm down after about a week. If you still have swelling after two weeks or the nodes grow rapidly then the advice of a doctor is needed. If the swelling moves to your collarbone or towards your neck, or your skin becomes red and inflamed then a doctor will be needed as well.
They swell when they are fighting an infection. Sometimes we aren't even aware we have one, but if you are feeling at all tired or fatigued, or running a fever, those would be symptoms. Even if there are no symptoms you know something is going on by the fact the glands are swollen. You need to have it checked out by a doctor.