It is very possible that a false negative urine test was obtained, so, your physician gave you Bactrim because of your symptoms characteristic for urinary and bladder infection.
Acctually, UTI treatment should not necessarily be based on the actual bacteria count.
If a woman has symptoms, even if bacterial count is low or normal, infection is probably present and antibiotic treatment should be considered.
It is possible that your urinary infection has relapsed, and in that case antibiotic treatment is prolonged for additional two weeks.
Although, Bactrim is effective in treating most of the cases of UTI's, still, there are some bacteria that are resistant to this drug.

See your physician and talk about continuing to take antibiotics!

Best wishes!
Marija