Most lumps behind the ear are due to infections or benign tumors of soft tissue origin. 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 out for a few weeks to see if it changes in size. If it gets smaller, it was nothing. If it happens to get bigger, then get it evaluated from your doctor.An FNAC of the lump will give a clear cut picture.You may take some pain killers or some anti-inflammatory drug if pain is there.
Nothing can be said with surety without an actual clinical examination.Let us know if you need any other information.