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 to see if it changes in size. If it gets smaller, it was nothing. If it happens to get bigger, then make an appointment with your doctor. You may take some pain killers or some anti-inflammatory drug if you have pain.
I hope it helps. Take care and please do keep me posted in case you have any additional doubts. Kind regards.