I have a uti. been on antibiotics for a week. i now have pain in my lower back on left side. is this a symptom of a uti?

Maybe yes: pain in the lower back to the side could mean that the UTI ( a bladder infection, aka lower urinary tract infection) has moved 'upstream', or up into the kidney, causing an upper UTI, known as a pyelonephritis. These can be serious, so, would recommend going in to doctor and relaying your symptoms and concerns, to ensure this is or is not the case.