Who can I contact to get detailed information regarding my Mom's Medicaid and payments to the nursing home?

Are you your moms POA? Is your moms contract with the NH based on a fee that she would pay each month or on the amount that Medicaid would pay? Did the finance mgr indicate that anyone else had been shorted by Medicaid? Surely your mom isn't their only Medicaid client. Compare your moms bills to see if you could find a difference in items charged for from one month to another. Sometimes billing errors are mistakes in what was entered. I would ask the finance mgr to help me find the person in their organization that works with Medicaid since the finance mgr doesn't sound very helpful. Then ask that person to call Medicaid with you to resolve the issue. If Medicaid hasn't reduced their coverage and the NH hasn't increased their fees my guess would be that an error was made on the bill going to Medicaid.
Hopefully someone who has resolved issues with Medicaid will see this post.