Red, raised, itchy rash

Could it be caused by something you have handled in the garden? I get a similar rash when cutting down ivy if it touches a part of skin not covered by clothing.

Try asking your pharmacist.

There are multiple things it could be, as well as multiple things that can have set it off.
I think you will need to contact a GP.

Sounds like hives (or urticaria, medical term).
I have been dealing with this menace for 5 years, it began a day after the Pfeiser covid booster. Not saying that's what caused yours but just for context I know what it feels like.
Aloe Vera gel (kept in the fridge) helps a bit. And Cetrazine seems to be the best anti histamine, recommended by G.P.

I did have that once, about 20 or so years ago. I don't know what it was, never found out. I have a feeling it may have been some sort of inflammatory response as I later discovered I had an undiagnosed auto-immune condition I'd probably had all my life. I had quite a bit of stress at the time.