The most common causes of such types of lump behind ear are due to the local infections like
,ear infections,recurrent scalp infections,dandruff
due to seborrhic dermatitis
etc.You should consult your doctor and get proper antibiotic prescription after a proper examination.Once the cause is identified,treatment becomes easy.
Since you have a scalp itching,it could be a case of scalp infection or seborrhic dermatitis.
If your lump doesn't disappear even after a proper antibiotic course or it increases in size,then you should get a thorough investigations to look for other causes like tuberculosis
or lymphomas etc.