if it is hard as you say , and behind the ear this means that this is infection of mastoid sinus bone behind the ear due to infection or old perforated ear drum.
this one , also may be simple scatch to the scalp that lead to enlarged lymph node you need to differentiate between the two , both are treated with antibiotics and take 3-5 days to disapear if the antibiotic is microbe sensitive ,if the have problem as ENT specialist for advise