Hello and hope you are doing well.
The rash could be Hives, which develop in response to an allergic reaction, by the release of histamine. The allergen can be medications, food, extremes of temperature, exercise and viral infections to name a few. So, therapy lies in resolving the allergen and anti-histamines. So, Allergy testing may be worth looking into. If the present anti histamines are not very effective you can consult your doctor for long acting ones.
Hope this helped and do keep us posted.
It sounds like you may have an allergy to dust mites, which are prevalent in bedding. Wash your bedding in hot water at least once a week. Try taking Zyrtec a few hours before you go to sleep. I suggest going to an allergist and being tested.