I had one a few years ago that was able to pee fine as a baby, but the bigger she got, the harder time she had of it. It turned out to be basically a birth defect, in that her urethra didn't grow as fast as the rest of her body did, and was actually leaking into her abdomen. She became very sick, septic, and died from the infection.
Considering other more treatable possibilities - was this fawn given any high doses of antibiotics or other meds over the last month or two? Many drugs eliminate through the kidneys, and there is a possibility of some damage if that is the case. Hopefully, not severe, and treatable.
Or, like Susan suggested, a kidney or bladder infection or UTI might be easily treated if you can get a sample and diagnosis.
Just know that sometimes nature can be cruel.
Good luck -