I Have A Strange, Red Rash That Sometimes Gets So

I have a strange, red rash that sometimes gets so itchy i can't stand it that runs down the top of my footbone about a half inch in width and about 4-5 inches in length. Most days it's just ugly; some days it gets a dry top layer; occasionally it itches so bad i just want to rip it off - benadryl spray helps the itching, but doesn't always arrest it. i had a melanoma taken off my back 20 years ago (i'm 63). One spot of this rash that i itched a lot turned dark brown and seems to be tan and fading now; i'm getting a few spots up my legs about the size of a children's aspirin or smaller. i have no idea what this is or if i need to be worried. it first appeared about Feb. of 2015.