I have had an itchy sore on the back of my leg just below the knee for a LONG time. It sounds exactly like the ones you describe. I have no idea what it is but a clue might be that I had a terrible (really terrible) case of poison ivy a few summers ago that took months and months to heal and left residual spots for more than a year.   I looked it up just now for the same reason you apparently did (is it the big "C"?). But now I'm back to wondering if it's just my body continuing to defend itself by producing (too much) histamine....basically an allergy to something that's no longer there....hope this helps.