Could a fall cause more decline?

Has the incontinence resolved? If not I would be suspecting spinal cord injury as vstefans mentioned.
She could have sustained a concussion if she fell on her butt and the force travelled up her spine to her head. No simple answer. I would put my check mark next to "all of the above"

Often the decline comes before the fall and that is why they order the imaging. The CT scan has to be compared with previous images. A finding of " no significant changes" is not the same as "normal". Something has changed and I would insist on a Neurologist, a GOOD neurologist taking a looksee.

A fall can be very scary and traumatic so both bowel and urine may be triggered by the scary incident. Her mental state is likely from the emotional trauma of the fall, panic, dr visit and ER visit. Continue to monitor her, does she talk about the fall, does she continue to be shaken up about the incident? If not, and she is rested and ok and her mental state is still odd, then call her dr and insist she be re-evaluated. They may have missed something or she could have a small bleed in the brain overlooked.

It's may be hard to tell which was first the chicken or the egg. She might simply be falling and incontinent because her Parkinson's is worse, or she could be worse because the fall was scary, or worst case scenario, she sustained an injury that did not show up on the CT scan. Did they at least x-ray her spine also? Is there any change in her ability to move or feel her legs or arms? Any change in her blood pressure (might be lower initially if spinal cord hurt at all).