The lump behind the ear on the head may be due to the trauma inflicted to the ears.What is the size and constituency of the lump (soft, fleshy, firm, rubbery, or matted).Is it movable or fixed?
Most lumps in that site 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 it out for a few weeks to see if it changes in size. If it gets smaller, it was nothing. If it continues to get bigger, then make an appointment with your doctor and get biopsy and MRI done.You may take some pain killers or some anti-inflammatory drug if you have pain.Also apply some warm compresses on the lump.
Hope it helps. Take care and pls do keep me posted on how you are doing or if you have any additional queries.