I've had several allergic reactions to medications. When I was 15, I was on Dilantin for seizures and got an allergic reaction to it 2 weeks later. I got a skin rash that spread all over my entire body in a matter of hours. My temp went up to around 105+. I ended up in the ER and almost died! It almost killed me but it also cured me! I never had any more seizures again (with no more meds). Now I am very aware of any changes going on when I take a new med. I am also severely allergic to sulpha as well.
Allergies can crop up at any time to anyone even if you haven't previously had allergies. They seem to get more prevalent (sp?) as we get older too.