What Causes Bruising And Swelling Behind My Ear?

For the last three weeks, I've felt what seemed like a bruise behind by left ear. I have had no trauma to that area. However, in the past few days, the area will swell and feel moveable; then it immediately goes back down--as a small ballon inflating/deflating. The pain is minimal and local. Slightly tender to the touch, though most of the day it is not tender to the touch. What could this be?