I have a uti and a yeast infection. i have been taking my medicine for it. it don't sting as much when i potty. it stung the next day really bad. why?

Depends on timing...: How long has it been? Symptoms take a few days to clear up, and may vary in severity. If the general trend is improvement over 2-4 days, probably all is OK. If more than 4-5 days, maybe the diagnosis was wrong or you need a different antibiotic for the UTI. If sexually active, STDs (e.g. herpes, gonorrhea) can cause similar symptoms and need different antibiotics. Check back with your doctor.