I once came down with a really bad fever, sore throat etc. that felt like flu. A fortnight later I was still feeling bad, so went to the doctor, who prescribed Penicillin - something quite usual given my history of throat infections and tonsillitis. For the first time ever, I came out in the type of rash that Eurox has described. Went back to the doctor and saw a locum who asked me if I'd had glandular fever. I said no, I hadn't, and she went on to explain that the GF germ can sometimes react with pencillin and cause this sort of rash. We went through my fever symptoms again, and she was pretty convinced that was what I'd had.
Just a thought.
On the other hand, it could just be an allergic reaction to one of the ingredients.