What Causes Urinary Retention After Undergoing Hysterectomy?

I have had bladder problems since my hysterectomy 4 weeks ago, at first I had to be re-catheritised for a couple of day as although I had good urine output I was retaining about the same. I have had a UTI since, I have had 3 courses of different antibiotics and told to see my GP again next week. I am worried that maybe I am still retaining and wonder why the tablets aren't making any difference. I am drinking lots of fluids when do yo think is a reasonable time to be concerned it's not clearing my GP just said its early day, but it is 4 weeks and I think a slight improvement should have happened by now