Rash at Night

I have been getting a rash  (hives) at night on my legs, arms, back.  It is accompanied by itching.  By the time I wake up in the morning the rash and hives are gone. I haven't changed any soaps, detergents, and I am wondering if anyone has any ideas of what this could be.  

[Dr_Aparna]

Hi,
Hives are red, itchy welts or swellings on the skin that often come in clusters. Hives can appear anywhere on the body, cropping up either in one small area or covering large patches of skin. They most often occur as part of an allergic reaction.
Hives usually appear suddenly and go away on their own in a matter of days, hours, or even minutes.

Hives are often caused by a hypersensitivity to:
•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)
•Animal fur
•Insect bites or stings
•Exposure to heat, cold, or sunshine.
Other potential triggers include physical exertion or exercise, stress, illness, chemicals, cosmetics, textiles, and pressure from materials rubbing against the skin.
Sometimes difficult to pinpoint the exact cause.

Comfort is the main goal of treatment, since most hive outbreaks are relatively minor and short-lived. To soothe itchy skin, try cold compresses, calamine lotion, a cool shower, or a tepid bath.
Try an over-the-counter antihistamine to reduce your body's response to the irritant and to relieve pain.
ref:http://www.myonlinewellness.com/topic/hivestreatment