Can Credit Collection agencies set a minimun payment amount? What are my legal rights by trying to make good.

I have made payment arrangements for some medicals bill that are 2 years old. For one month I had to change my amount, but still want to pay something. Then resume higher payments the next month. Then all of a sudden my bill grew, as she found where i had owed more. I really can not dispute this. Sometimes you take the wrong road and play hell trying to get on the right path