Hello: I Am Taking IC SULFAMETHOXAZOLE TMP DS TABLET, I

Hello: I am taking IC SULFAMETHOXAZOLE TMP DS TABLET, I AM TO TAKE ONE TWICE A DAY. I TOOK THE FIRST ONE FRIDAY NIGHT, TWO YESTERDAY AND ONE THIS MORNING. THIS AFTERNOON I HAVE GOTTEN A SMALL BLOTCH (2 X 4") THAT BURNS ON THE SIDE OF MY NECK, AND A SMALLER ONE STARTING ON THE OTHER SIDE OF MY NECK. I HAVE TAKEN THIS MEDICATION BEFORE WITH NO REACTION. COULD THIS BE A REACTION TO THIS DRUG EVEN IF IT DIDN'T BOTHER ME BEFORE? I DO NOT KNOW IF PREVIOUSLY IF IT WAS DS OR NOT. THANK YOU. 'O'