Swelling behind ear
The most common swelling right behind the ear
over the mastoid bone is some kind of cyst.
Sebaceous cysts are common. Another swelling could be an enlarged lymph node as a reaction to some prior infection in this area. Both of these can fluctuate in size from month to month. Lymph nodes feel firmer, solid and tend to get smaller over time. Cysts feel softer, compressible and get larger over time.