*******ly charging legal fees on an account paid on time

Had a medical emergency in October 2022, my medical aid had no available funds when Dr submitted their claim however received communication from the Dr regarding funds not paid on the 31 October 2022. I then made payment on the 2nd of December 2022 of the full outstanding balance and the Dr confirmed and acknowledged receipt of payment however now getting calls from a legal firm harassing me of the outstanding payment. I don't understand why my account got handed over after i made payment.