Well, UTI is the most obvious answer, but have you also had a specimen of stool sent for culture and ova and parasites? I would recommend having the stool checked.
Another possibility could be a systemic yeast infection. If that is so, antibiotics would only make it worse. A systemic yeast infection would be treated with Diflucan an oral drug.
You could try adding yogurt to her diet regularly (about twice per day) Just don't buy the sugary kids "yogurt" you need one with active cultures and no or very limited added sugar.
Another diet supplement would be cranberry. Cranberry juice cocktail is full of sugar and will worsen yeast, however cranberry extract can be found in a concentrated supplement form. Any health food store or Whole Foods would carry it.
Could she be lactose intolerant? Sometimes "watery stools" are caused by lactose intolerance which in turn leads to more cross contamination of her urinary tract. You could try eliminating milk from her diet use Lactaid instead and see how she reacts. My daughter who is lactose intolerant at first did not like the lactaid so I added chocolate sauce. Now she hates the taste of regular milk and only wants her Lactaid. (even without the chocolate sauce) My daughter, like many others who have lactose intolerance, can eat cheese it is only milk which causes her symptoms.
Additionally, keep her well hydrated to decrease the acidity of her urine. Consider using a "peri-bottle" to rinse her peri area when possible after urinating. Also monitoring her wiping after BM to be sure she is wiping front to back and meticulous with her cleaning. Residual stool can be very irritating to the skin and can lead to UTI when tracked into the genital area. A light skim of A and D ointment applied to the rectal area and genital area can be protective and soothing. (obiviously, apply to genital area first then rectum to avoid cross contamination)
Additionally, when she does void, make sure she is pushing her pants all the way down to the ankles and opens her legs slightly to allow the urine to flow quickly away from the body. And I'm sure the pediatrician has already told you avoid bubble baths.
Assuming your daughter has already had a full bladder workup including looking for urinary reflux and urine retention; I would also remind your daughter not to hold her urine too long as she will enlarge her bladder and make it less effective at emptying.
Once she does urinate. If she states she has to urinate again 5 minutes later ask her to wait. She is probably feeling a little spasm and straining to urinate further will only cause more spasm. Assure her the feeling should go away in a short while.
You could also try delving a little into her psyche. Sometimes with small children little worries which they keep to themselves can balloon into obsessive thoughts which could be alleviated if they share them with others. My six year old who had similar symptoms was really just obsessed with having an "accident" so she just kept going and going until she was having symptoms of UTI secondary to the straining.
Sorry I went on and on, but as you can probably tell we've been through a similar situation. I hope these suggestions are helpful.