My 20 Month Old Granddaughter Has Been On And Off

My 20 month old granddaughter has been on and off sick for the last few weeks. All over body rash, fever then hives and a fever with no appetite. Blood work is normal, lung X-ray is normal and she doesn't have a UTI. She hasn't eaten in 6 days has no energy and has been sleeping all the time. What could this possibly be. We are all getting very worried.