Overpayment for youth allowance

I am on the youth allowance and for medical reasons I had to drop down to part time study. I went in to the service centre and I uploaded a medical certificate which was approved.

I’m now getting a call saying my medical certificate expired on July 1 2024 and my studies started again on July 28 2024 so I have to pay that back. I’m a bit confused because when I called them about my medical certificate they told me that the date has to be till the end of the current semester, no one told me it had to cover the holiday period as well. She told me that it’s on me because I didn’t get another medical certificate. Does anyone know what I can do in this situation?