Unexplained rash?

Mary • Mom of 2 boys one 15, other 5 so here is hoping for my girl
I've slowly developed little red rash bumps they go up and down both arms and itch like crazy!  I've not changed my food, drink, laundry, clothing or perfume or lotions so I'm worried.  Other than constant itch they haven't cause any they noticeable issue.  Taking Benadryl and using hydro cortasion cream not helping what now?