This happened to me about a month and a half ago...for a few days I thought I had shingles (karma for having chicken pox 4 times as a kid). Rash on the forearms, and just above the knees.
During the itchy & scratchy phase (which was really bad at night), the skin was scratched to the point of bleeding..and in spite of keeping everything clean, I got a major infection in both arms, they swelled up bigger than a roll of TP.
Doc rules out shingles, gives me antibiotics, swelling and infection goes down, but suddenly I get large welts all over. Turns out I am allergic to the antibiotics!
It's been slowly healing up, doc recommended antihystemines to reduce the allergy effects (just over the counter stuff they sell for runny noses etc).
I think it was (in my case) the laundry soap...I'd changed formulas (same company), but was probably over-using the amount needed. I cut down the serving size of the soap by half, and it has been s l o w l y recovering.