How To Get Rid Of The Rash On The Fore Arm?

my 11 year-old daughter has had a rash on her forearm for about 2 weeks. ther is no regular pattern but the bumps are in clusters about an inch and a half long and half an inch wide covering about a 4 inch square space total. The bumps look a little blistery and tiny little scabs are forming at the top of some of the bumps. Today for the first time, the rash has spread to the under side of the forearm of her other arm. This outbreak is a 1 centimeter by 1 centimeter. We have been treating with hydrocortizone and calamine lotion for itching but rash has only grown slowly rather than disappeared. What could this possibly be and how should we treat it to get rid of it?