Matthew,
The problem you mom has could be due to genuine diarrhea, either as a reaction to medication ,or as an ongoing gastric infection or similar, OR it could be related to constipation which is well known in people with PD. This can often result in leakage, and I would hope that it has been eliminated from the suspect list by the hospital. If not, I would say that it needs to be looked at. It is not at all unusual, and is a very real hindrance to quality of life, as well as giving a lot of discomfort. I would not have thought that sinemet along will cause this problem, but wearing off and on could cause a digestive system to work a bit intermittently. Sorry can't be of any more help, this is about the limit of what I know as a lay person, but hope that you are able to find some kind of solution.

Best wishes
Lindy