I Have A Rash That Shows Up And Disappears All

I have a rash that shows up and disappears all day long and tends to be worse at night. Its always on my thighs back neck or arms. I been taking benadryl and using hydrocortisone cream and it doesn't seem to help. The only side affect left over from the rash is little red bumps but that's only if I get it on my neck.