Oh dear, that is terrible. I have heard of a fistula but I thought it only came from a difficult labour/childbirth. Maybe do a google on it. (Has the doctor determined that it is urine coming from your vagina and not some kind of discharge?)
The main symptom was urine leaking from my vagina. My bladder was accidentally injured and repaired during the hyst, so I came home with a cath for several weeks. However, after that was removed, I kept feeling damp and like urine was coming out my vag...because it was!
Call back your dr ASAP and let him/her know what is happening! There are several tests to see if you have a fistula. The first/easiest is to do a "tampon" test...however, do NOT do this unless you have permission to have sex... you don't want to injure yourself or cause a setback to your recovery.
You can buy an OTC product that turns your urine orange (AZO or pyridiam). In my case, I had been taking vitamins which colored my urine bright yellow so I did not take the OTC product. Anyway, after inserting a tampon, leave it in until you need to urinate. Remove it before urinating and look to see if there is any kind of color on the tampon. In my case, it was bright yellow because of the vitamins.
Your gyne or a uro can also due a test where they put blue dye in your bladder via a cath...They pack your vag with gauze, and then after 15-20 minutes remove it to see if there is any blue dye on the gauze.
In any case, please CALL YOUR DOCTOR! I had a constant bladder infection which required antiobiotic treatment while I was waiting for repair. Urine should NEVER come from your vagina.
If you have any specific questions, please feel free to e-mail.