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. From your symptoms
it can be a boil or an abscess.
You should keep the area clean and apply some topical antiseptics. Also wear loose-fitting cotton clothing and use an antibacterial soap or mild soap like dove. Avoid any kind of cosmetics.