I went to dr yesterday and found out i have kidney infection. how long before i start improving. i'm hurting in my stomach, back. taking bactrim (sulfamethoxazole and trimethoprim). ?

Pyelonephritis: As long as the bacteria causing the infection is sensitive to Bactrim (sulfamethoxazole and trimethoprim) you should be improving soon. If you have vomiting or unrelenting pain it is a good idea to notify your physician. Some patients who develop kidney infections require in hospital management with IV antibiotics if they are unable to tolerate the oral treatment or have progression on oral antibiotics.

Kidney or bladder?: Most people get relief from bladder infections pretty fast. It's possible the bacteria is resistant to bactrim (sulfamethoxazole and trimethoprim). You might also have a more complicated kidney infection or a kidney stone. Any fever? I'd check on the culture results.