Behind the ear there is skin, fat, cartilage, bone, nerves, blood vessels, Lymph nodes, and salivary gland tissue (a portion of the
parotid salivary gland extends behind the ear
). A bump can develop from any of those structures, and could be due to a tumor (benign or not), an infection (Lymph node or skin), a swollen muscle or salivary gland, etc.
My suggestion would be to wait it out for a few weeks to see if it changes in size. If it gets smaller, it was nothing.