Thanks for writing to H.C.M. for your query.
I have gone through your query regarding lumph behind the ear, it is freely movable, tender to touch.
Considering all features of lump as you explained it appears to be -
(1) Enlarge lymph
(2) Sebaceous cyst
To confirm the diagnosis you should go for FNAC from lump.This FNAC will tell weather it is-
(1) Infection of lymph node- may be acute or chronic like tubercular.
of lymph node may be benign or malignant.
(3) Sebaceous cyst ( may or may not be infected ).
Depending on diagnosis it may be treated.