I would honestly wait another day or two to see if it breaks...especially if she doesn't have other symptoms. Both Fifths Disease and Roseola are rashes (very common in children) that appear after 3 or 4 days of unexplainable fever. You can do a search on both to get more information. Just be on the lookout for a rash. With Fifths Disease, her cheeks would get really red and look like they have been slapped (but you'll definitely know it is a rash). :)
As far as a UTI is concerned, many times for small children, the symptoms are not as clear as with an adult. As a general rule, make sure that you always wipe her from her front to her back so as not to introduce excess bacteria into her vagina. If it IS a UTI, getting a urine sample can be very tricky...I have heard from another mom that she first gently cleaned the area down there (this is a must for a urine sample), then she and her husband ran some warm water in the bath, stood their daughter up in the warm water and had the cup already in position. They were able to get a sample that way.