UTI or puberty?

[Timtims]

12yo DD has been havinga pain in her bladder-ish area for a few weeks. She says the pain is worse lower down during and just after she has a wee (apparently burning not stinging), but that she has a dull ache a bit higher up quite a bit of the time.
The GP tested for a UTI but negative result (although not sure sample was done properly 're mid-stream urine ).

So im wondering if
A. She should do another sample to be sure.
B. It could be something related to puberty ( she has some pubic/armpit hair, and some discharge, plus early breast growth, but no period yet ).

At a bit of a loss, so any help appreciated.

Make sure she's actually drinking enough (non-fizzy, non-citrus, non-caffeinated) fluids. Depending how big she is for her age, enough is probably somewhere between 1.5 and 2 litres, which needs to be spread out through the day, not just catching up after school. Obviously ban bubble bath. Is she constipated at all?

No not constipated.