Not running for a few days

I've curtailed my running for a few days, due to a large "blister", or ulcer, on my left leg. The ulcer is, more or less, a square about 2 inches on a side in which the skin has been removed, leaving the bare skin underneath. It is quite painful when I walk, although the pain seems to be subsiding. I don't know why the skin has been removed on the ulcer. It might be due to the compression stockings I've been wearing, and I've temporarily removed them for a few days.