Is the recipient responsible for overpayment when they had nothing to do with the reporting?

I went through stage 4 ovarian cancer and while I was out the benefit specialist reported my hours directly to the UNUM agency that she received from our Human Resources office. I was out from July 19th, had surgery for a total hysterectomy on July 20 and was out until UNUM I would be covered until September 19th, medically I was able to return on September 4th and did minimal hours until September 19th, which our benefits coordinator reported. So the last benefit I received was not the full amount and I thought was from the minimal hours reported. Now they are saying I was overpaid by more than $700.00. I had no knowledge was being reported back and forth from benefits specialist, Human Resources and UNUM. It's not easy living with cancer and to harass person because of their error is totally unacceptable. Can someone help me with this?