Employment Law Questions? Ask an Employment Lawyer.
They are correct in setting up a meeting to discuss the matter with you. There isn't any law against that. In fact, your union rules probably require it prior to employment decisions.
They can legally request a doctor's note because you took time off that was medically related. They are more in the right to ask for this, given that you took the sick leave immediately following a different form of leave.
Your protection here comes solely from your collective bargaining agreement and union grievance process. You don't have any sort of protections based on the bereavement or sick leave, because neither federal or state law required the employer to grant either in this instance.
If your collective bargaining agreement doesn't allow the employer to ask for doctor's notes then that certainly can be a defense here, but that's what you're going to have to look to...the CBA and union help, because nothing in your facts indicate any illegal action by the employer to this point.