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.
Since it is causing numbness on that side of the head,so it must be pressing on the nerves supplying that area.My suggestion is to make an appointment with your doctor and get it evaluated.You may take some pain killers or some anti-inflammatory drug if you have pain.CT and MRI head along with biopsy of the lump may be needed to confirm a diagnosis.
Hope it helps.Take care and pls do keep me posted on how you are doing or if you have any additional queries.