Herpes would not appear so quickly and probably not as a widespread rash as you describe. When you say welts though, do you mean red rash with raised bumps like a mosquito bite? No fluid or anything inside?
If that's the case, it sounds like an immune allergic reaction, try some antihistamine tablets and see what happens. Of course follow up with a doctor when you can.
In the time since I posted the question a few small pimple like bumps have formed and the redness has gotten darker and more concentrated. I popped one of the small pimples since I read that herpes blisters bleed when popped and the pimple didnt bleed so Im hoping its a positive sign. I took an antihistamine which helped with itchiness but not with the redness