Unfortunately rashes are the worst to try to figure out.
New soap? Laundry detergent?
Keep a food diary.
Often artifical fragrances are culprits, I can't use anything with any perfume except natural pure essential oils with no other stuff. Hard to find except in natural food stores. Not dish soap, any cleaners, clothes detergent, shampoo, soap, nothing. I have to read the fine print. If I use any household cleaners with perfumes (and some other ingredients) much of my skin turns red, bumpy, itchy and burning for days.
Also shampoo which contains sodium lauryl sulfate, sodium laureth sulfate (not sure of spelling), methyl paraben or or propyl paraben cause severe itching. Currently I'm using Kirk's Castile Soap for shampoo and rinsing with a quart of water and 1 T. white vinegar added.
I wasn't always like this, it got worse gradually.