Hi
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It is difficult to comment without examination. Lump behind the ear could be an abscess, folliculitis (hair follicle infection), lipoma (localized collection of fat), a cyst, an inflamed lymph node or a neurofibroma (localized harmless swelling in of nerve and connective tissue). You may not have been sick but local infections like dandruff or a dermatitis too can cause a lymph node to swell and be tender. Please consult your PCP and get this examined. Take care!
You may be having some infection in your ear, sinuses, etc. Even a tooth abscess could cause this. See your PCP.