Unfortunately, without the ability to actually see this skin eruption that you are describing, it would be very difficult, if not impossible, to blindly offer an accurate diagnosis. A diagnosis is really essential when it comes to treatment. Knowing your medical history will also help solve this mystery rash.
Based solely on your description, this could be an allergic reaction, perhaps urticaria (hives). Hives are intensely itchy, comes on suddenly, and if the allergen is still in your midst, it can continue to persist/spread.
Regardless of the cause (which can be very challenging to find), the itchiness can be treated with an antihistamine, like diphenhydramine (Benadryl) or other over-the-counter medications in this class.
Seeing a medical provider is important, since many things can mimic hives.
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