Bladder Infection. Frequent Urination In Child. Has Dribbled Undergarment. What Is It?

Hello My dauhhter just turned 3 since then she has had one bladder infection .. She has been potty trained since 2.. When I say that I mean all night day.. No problems in the last 2-3 months she has been having accidents, running to the bathroom saying pee.. Like she can t hold it.. She gets upset. We don t know what is causing it.. She changes her underwear and pants because she in embarrassed.. I have taken her to the pediatrician .. One time he tested her urine said it was positive.. Sent it out came back. Negative..now he sent it out again.. He is waiting before doing an ultrasound .. He may think its voiding.. I just have a feeling it s something.. She constantly wants to drink were trying to control her intake of liquid. She pee walks away, and has to pee again. There is constant dribble like she can t control it.. She goes every 15 min and we still have about 8-10 dribbled underwear everyday.. School home.. We are frustrated.. She plays is happy. Has regular bowel movements eats a lot.. Please help need another opinion.. C