I had similar aged about 11-16. I had endless tests done on my urine and they all came back clear with the exception of some protein or blood in it. It got to the point where the doctor basically accused me of making it up to get out of school (despite the fact that I'd never missed a day ).
I ended up having a cystoscopy and several long courses of antibiotics to try and clear it up. Cystoscopy was normal and they didn't find a narrowing of my urethra or anything but after that I don't think I had one for nearly 6 years. My consultant suggested it was possibly just helpful to have my bladder filled and emptied with something sterile for a change. He also suggested that the reason for me getting them so much was because I don't go to the toilet that often- I just don't feel like I need to go unless I am totally desperate!
They had come back with a vengeance up until a year ago when I started taking D Mannose tablets to try and support my urinary tract. They are the only real change I've made (having struggled since primary school age I think I've tried just about every trick in the book!!) and I've not had a flare up for over a year now- massive change from having at least one a month! Definitely worth looking into as they really can't do any harm and if they help just a little then it can't be a bad thing! Look up Waterfall D Mannose (Sweet Cures) as they have a lot of fab information about it on there! Then buy it off eBay from Harvey Nutrition like I do haha!
As boring and as crap as it sounds your DD will have many patronising conversations with consultants suggesting that she does all the usual correct things- wipes front to back, pees before and after sex, washes with only water, empties her bladder frequently, drinks plenty of water, reduces sugar etc etc. This stuff is really boring and makes you feel about three but I do really find that trying to keep it all in mind at all times seems to just give you that edge on fighting it. I seem to recall a conversation with a MALE urologist telling me how to pee correctly
Hope she gets sorted soon- it really is utterly shit feeling like that all the time