I took antibiotic for strep throat which didn't work. doctor switched me to clarithromycin yesterday. today i have one hive. should i be worried?

Prob not: One red, itchy, raised spot (hive) is not usually a sign of a systemic alleric reaction. If you are having an allergic reaction, you will know it shortly, the one hive will become many. If that happens, stop the antibiotic, take a benadryl, (diphenhydramine) and call your doc or head to the er.

Double jeopardy: Strep infections are usually sensitive to amoxicillin. Did you have a culture before starting treatment? If you didn't respond to the Amoxicillin there is a good chance that you had a viral infection by starting another antibiotic you always run the risk of developing an allergic reaction to it, such as developing hives.