I had a horrible rash my last pregnancy that started around 21 weeks and eventually went away around 25 weeks. My doctors were unsure of what caused it but recommended me to take benedryl. Have your ob check it out as it could be something wrong with your liver or just your body having random pregnancy hives.
Drink a lot of water. Also let your doctor know he will give you a cream that will help. Its an easy fix. Good luck :)