Okay, my experience of this is from 2005, before Part D and other changes...
My understanding is that after hospital care, a patient can go to skilled nursing where Medicare pays 80% and the secondary (Part C) provider pays 20% for 20-25 days (this may vary by state and time of event). Medicare will continue to pay the 80% up to 100 days from date of admission. Typically the patient or family pays for non-covered expense. Most facilities allow for a payment schedule.
Medicare will only pay for so many days of PT and OT, which must be requested by her physician for each group of sessions. Again, the provider should bill Medicare directly and her Part C should pick up the rest.
You can ask for a detailed statement from each facility and provider that covers a range of dates, showing charges and payments on her account. You should probably do this anyway to document what's going on. Also (I found out the hard way) providers seldom send a statement when the balance is paid in full unless you specifically request it.
She should be getting statements from Medicare and her Part C company, showing what's been submitted and what they paid. If they are paying providers you've never heard of, note the dates and ask the facility where she was at that time to see if they authorized a consultation or treatment you didn't know about.
Hope this helps.