Can a collection agency ask for more money on daily interest charges after the payment was made in full?

2/14/07 - Paid the collection agency full amount

2/26/07 - Received a call today saying that there is still daily interest charges balance remaining that I need to pay from 1/24/07-2/14/07.

When I spoke to them on 2/14/07, they told me that if I paid the amount in full, that is all that I had to pay.

Do I still need to pay these interest charges? Why didn't they tell me that I would be receiving a call later of additional charges?