Burned my ankle 3 weeks ago been on bactrim (sulfamethoxazole and trimethoprim) and now keflex burn site swollen and red still leaking fluid, very painful to walk, what should I do?

Get to a doctor. Urgent care clinic, your dr or even an er might avoid major complications. Do it now - thinking about it will not help it and more than enough time has passed already. Legs are foundations. Take care of them - they'll take care of you for years to come.
See your doctor. For follow up care.