What Causes Itching And Rash On The Body?

I have been getting a hive type rash every night. All traces of the itching and rash are gone by morning, yet make me mental at night! Sometimes will be certain parts of body, sometimes from head to toe! I swell so bad it looks like elephantitis! what could it be?

I am a 36 year old woman, and have not made any changes in my bath or laundry soap, food, etc .... and it only seems to appear and drive me insane at night.