Sudden Hot Itchy Rash at Night

Once in a while at night while asleep I awake to a hot sensation all over and itchy skin from head to toe.  You can actually watch my skin turn red in the mirror as this progresses.  I take an antihistamine and roughly 30-45 minutes later it clams down and I can return to sleep.  This does not happen every night, may only happen once a month.  Had a hysterectomy over 10 years ago, and am not on hormone therapy.  I am 53 years old and not sure this is related to menopause (since I had a hysterectomy) or what it can be.  The next morning after one of these rashes comes about, I am run down and extremely tired.  Got any ideas?  I have no known food allergies.