Help!!! UTI problem im desperate!!? Help! I've had a UTI since last Wednesday &they finally gave me Bactrim DS antibiotic on Tuesday this week ..? I'm still hurting in my bladder and have the uncomfortable feeling of the UTI but i now am having upper leg soreness and some dull pain in my lower back kinda think its turned into a kidney infection .. I just wanted to know how long it should take before i should start to feel better after starting the antibiotic ? Its been 3 days on it and im not feeling any relief..