Rash all over body starts on head itchy but bearable, then spreads all over body rapidly first looks like nettle sting but then turns extremely red ?

Urticaria/Hives: Rarely, a reaction to medication, food, food additives, insects, parasites or infection is identified as an underlying cause of chronic hives. But in most cases, the cause of chronic hives is never identified, even after testing and monitoring symptoms. Heat, cold, pressure, sunlight or other environmental stimuli may worsen chronic hives. Certain meds such as aspirin, Ibuprofen can cause hives.