What Causes Recurring Lump Behind The Ear?

I keep having a cyst like ball behind my ear appear evey once in a while. probbaly once every month or two. I thought it was a pimple that went away, but seeing as though its come back at least three times now in the same spot, should I be worried? I cant really look at it because its behind my ear, but it hurts like a pimple would?