What Causes Itchy Bumps On Legs, Buttocks And Arms?

My 2 year old has had a tiny red bumps(sometimes they aren t red and are slightly raised) on her buttocks area for almost 2 weeks. I took her to urgent care a week ago and they said it was contact dermatitis. A week post the appointment the rash is still there and now it s itching her and I have notice more flesh colored bumps up and down her legs and arms. Sometimes the rash looks like its almost gone and then the next morning it s red and irritated again. I just bought clear and free detergent hoping it may help. I have been using the same detergent for a year now but if this in fact I contact rash, detergent is the only thing I can think of that may be causing it.