It is difficult to comment without examination. Lump behind
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.
Consult your PCP and get this evaluated.
Hope this helps.