I am sure a have a urinary tract infection and my lower back started hurting so i think it spread to my kidney, i just bought some medicine for the ut?

Need urine culture: If the infection has spread to your kidneys, you need to get SPECIFIC treatment immediately. Your doctor needs to get a urine culture to find out exactly what type of bacteria is causing the problem and then use an antibiotic that will kill that specific organism. Nitrofurantoin and sulfamethoxazole/trimethoprim are generally good. Treatment must be long enough to fully clear the infection.