What Causes Recurring Rashes All Over Body?

2.5 yr old female with recurring rash covering majority of body. Rash is patchy with sandpaper texture, worse when exposed to heat & comes & goes. Began over 2 months ago. No fever, no slapped cheek appearance, no other symptoms. Took to pediatrician, strep neg, no blood work done. Ped said that it had the appearance of rash that comes with strep throat, but since strep was neg and no fever she decided it was fifth s. If indeed it was fifths, why is the rash still present over 9 wks later? Everything I have read says it can come & go for 3 wks. Hydrocortisone or lotion do not soothe her itching.we already use aveeno wash. She is miserable at times & usually has to have Benadryl at night to keep from waking up from the itching. Doesn t always work. Could it be something else??