What Causes Liquid Filled Bump Behind The Ear?

I have a liquid filled bump behind my right ear. It s not in the flesh part but right behind the cartledge-feeling-part of the ear....like if you stick your finger in the very outer part of your ear into the part that feels like a little bowl, it s behind that. It feels huge & tender & full of liquid. No head on it. Just feels like thin skin. Lasting for a while now...maybe a month. Not sure.