carmac

I had made an arrangement to pay $50.00 a month on a hospital bill, the bill was about $5,000 I have been making payments for 2 1/2 years, suddenly last week they sent my check back and when I called to see why, an *****le told me that it was not enough, that he wanted $100.00 a month, and that he did not have to accept any payments at all, so he scared me, so I am going to send what they want. I felt personally attacked I had never been late on my payments so how can they do this? do I have any rights I do have a job now but I am paying other commitments as well. Can anyone help me, with some good information Please.