Doctor Under Charging Me

I went for my wellness check up yesterday and there was a new person at the front desk. After I signed in she calls me to the front and tells me that for several years they have been under charging me for my co-pay and I owe them several hundreds of dollars. They said they need me to pay before I see the doctor.

I canceled my appointment and quit as a patient. She said this out loud in front of a room full of people waiting. It made it seem like I don’t pay my bills.

Can they do this? They told me a price and I paid it. Can they come back after years of under charging me and demand back payment for their mistake?