You may have a vaginal fistula that is bothering your urethra. I thought I had a UTI but all my tests came back negative, so I went to a urologist who noticed a little hole right outside my urethra. Previous to that I had my GI tell me that she felt a closed fistula that pointed toward my vagina.
After that, though, both the urologist and the GI wouldnt go further to look at the problem. They each said it was the other's issue But luckily it must have closed (probably due to a remicade infusion).
If your tests are negative you might want to think about not saying anything to your doctor, because if they think you have an infiltrate they wont let you get your infusion (because it lowers the effectiveness of your immune system). I know most people on here will think I'm wrong to say that, but that is what I would do. Now I only mention this paragraph because your post made it sound like the tests for bladder infection came back negative.
You could also have a fistula to your bladder. Regardless of where a fistula is, if it IS a fistula causing your problem, Remicade is the best course of action.