N.S.
I just went through a very similar experience with my 2 year old who was having recurrent UTIs one after another, she even got one while already on antibiotics. Anyway, when the results come back from the urine culture you will know for sure whether or not she has a UTI. The ultrasound will look to see if there is any communication going on between her ureters and other body parts, like intestine or her umbilical artery. If it does come back as a UTI you may want to request a VCUG. This is an x-ray procedure where a catheter is inserted and a contrast dye is then injected into your daughter's urinary tract. Then an x-ray is taken as your daughter pees to see if the valves in her ureters are functioning properly, or if they are allowing urine to back up into her kidneys. The most uncomfortable part of the procedure is the insertion of the catheter, and she may also feel a little uncomfortable when her bladder is full of the contrast, but once she pees it will be all better. (We just went through this procedure last week on my daughter) The ultrasound will not be able to tell you if her urinary system is functioning properly,which is why I suggest the VCUG. The ultrasound can only look at her organs and see if they look correct.
One other thing, does your daughter seem to have pain in her low back, I know it's hard to tell in such a little one. The tip off to my 2 year olds UTI was that she complained that her back hurt and was walking around hunched over with her hands on her back like a little old lady, poor thing. Anyway, the kidneys are located in the lower back, so if she has pain there I would think her kidneys are involved. That doesn't mean anything drastic is happening, just confirms infection really.
Hang in there, and get everything checked until you feel comfortable with the diagnosis.