After a week on bactrim (sulfamethoxazole and trimethoprim). is my spider bite getting worse or better?

I can't see ur bite: Sorry, i can't see your bite & you didn't describe it so i can't tell. Actually, you should be best judge of whether it's better or worse. If better, less redness & pain, right? If worse, more redness & pain! if you're still not sure, go see your doc or whoever prescribe your bactrim (sulfamethoxazole and trimethoprim). As always, take as directed & finish your course of medications. Don't save it for next bite.