Hi,
This is a case of hives, an allergic reaction. For the entire body to be involved, it must be due to a certain food or drug.
Hives are red, itchy welts or swellings on the skin that often come in clusters. They usually appear suddenly and go away on their own in a matter of days, hours, or even minutes. New ones may continue to appear until the condition resolves.
Foods (commonly eggs, shellfish, nuts, berries, dyes, or other additives), drugs (any drug can touch off an allergic reaction, although allergies to penicillin, sulfa, and aspirin are especially common), pollens and plants (nettles, poison ivy, poison oak, and so on) can bring on this kind of reaction involving the entire body.
If you can recall any contact with any of the above mentioned or anything different that you tried a couple of days before the rash, you could probably consider that the allergen.
Your treatment is along the right lines, continue with the Beanadryl, which is an anti-histaminic and also apply the anti-itch lotion.
Since the area of involvement is extensive, it may take a week for the rash to totally subside. To soothe itchy skin, try cold compresses, calamine lotion, a cool shower, or a tepid bath with a few tablespoons of cornstarch. This is to provide symptomatic relief.
ref:http://www.myonlinewellness.com/topic/hivestreatment