I have a hugh hospital bill -we pay on it every mth- so why they are sending us to collections?

we pay every month on this bill and the hospitsal called and said we need to fill out a form to quallify to make payments- we never miss a payment but did not fill out a form because we do not feel they have to right to tell us how much we can afford to pay- now they say that they are sending us to collections- I do not understand- we pay- what are my rights? we live in Marion county-in Florida.