Outstanding medical bill from 2022

I hope with this I will get assistance, I used to be a member on this med from 2016-2023, wherein I fell sick and had to leave my employer in order to take care of myself, now in 2022 I had an emergency birth delivery and according to me all bills where paid, recently I received a call from a Dr office and they claim the bill was not paid by the medical and that "they could not find me" and theres more additional costs to the invoice, from the 5th to date I have sent 4 email to the claims department, wanting to know if this Is real or is it a ****, as I also don't understand how can I get an outstanding bill 2 and half years later and I am nolonger a member to the medical, to date I have not received any feedback back from the medical and I seriously need assistance.