Insurance collection letter

I worked for a company that continued to supply our insurance after Hurricane Katrina. They extended the cut off date several times and all of my prescriptions were covered for many months after the storm. I would go to the pharmacy, refill a prescription, pay and that was it. It is my understanding that each time you fill a presciption, the pharmacy ok's the refill with your insurance company. I did as normal without any changes or problems. My prescriptions were always filled at the cost of only my co-pay. One year later I have just gotten a letter from a collection agency saying that these claims were in error and I owe them 250.00 for prescription cost. I bought them from the pharmacy (a national established chain), was charged a certain amount, paid it and that was it. How can they hit me with extra costs after a year saything that they were not covering my prescriptions when the prescriptions clearly went through?

You may argue that you relied on them and it is not your obligation to now pay; you could even go to small claims court if need be.