How To Get Rid Of Rash On Arms, Legs And Buttocks?

My granddaughter is 15 months old and has a blistery rash on her arms legs and buttocks. She had a slight fever a few days ago but now is gone, she is very irritable and doesn't want to eat much. She also has been having a running nose and a loose cough. Was wondering if you have any answers of what she may have and what we can do to help the rash. The rash seems to be spreading. My daughter is putting Lubriderm on the rash but doesn't seem to be helping. Any advise would be appreciated. Thanks