Collection agency given un billed debt

by Steven
(Portland)

I was in an auto accident and was uninsured. I paid both drivers directly for their personnel losses and expenses. I struck twp parked vehicles, I paid one of the insurance companies out right. The second claimed that the damage to the car was $3,604 dollars. I talked to them and was told that I could make payments to them. The next I was heard less than a month later was from a very rude collections agency. I began making payments to them and on the third payment I noticed that the balance owed was over $10,000 dollars. When I called and asked if this was a mistake I was told the insurance company had had additional expenses and I owed it. When I asked for proof of the expenses and an assurance that they would not continue to add debt I was told that I just needed to pay what I owe and hung up on.
Is this correct?