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This will almost certainly be cystitis, usually called juvenile cystitis at her age. This means she has an infection in the bladder. It’s fairly common. Treatment usually involves antibiotics and sometimes a medication to change the acidity of the urine but you would need to see a vet for that.
In the meantime to make her feel more comfortable you can give her cranberry juice. This causes a mucus lining to form on the bladder wall and also increases the acidity of the urine. Sometimes that will give fast relief. The dose for her would be 1-2 teaspoons. Mix it with food or into a small volume of water.
Continue to encourage her to drink water...that helps too.
I hope I have been of help.
Kindest regards, Peter