What Is The Dark Rash On My Upper Back Along With Headaches And Dizziness?

I was prescribed Trimethoprim for a suspected water infection, took one tablet and within a few hours my skin was very itchy and prickly but with no rash, so did not take any more. That was on Saturday, it's now wednesday and it has developed into a feeling of sunburn on my skin, and a slight rash has appeared on my upper back, also getting headaches and dizzyness. Is this still a reaction from the trimethoprim, or is it something else?