Medicare is the government created health insurance system. It is not a perfectly functioning system, although it usually works pretty well for most people. It does however have its flaws, and therefore private industry has created supplemental plans to help cover the areas where Medicare does not. A Medicare supplement plan is the type of insurance that individuals can buy when they want to make sure that they have everything covered in their insurance.
Rates on a Medicare supplement plan can and do change. As the individual who holds the policy becomes older, they become more of a risk to the insurance company in that they may become more likely to file a claim. Based on this fact, the insurance companies will raise the amount paid to maintain the insurance. They usually raise the rates on a one, three, or five year basis. This is all predetermined before you sign the policy. That is to say that you know up front before these rate raising are going to go into effect.
Unless specifically mentioned in the contract, usually there should not be a time when your rates go up that is not mentioned. These contracts have the authority of a legal document between you and the company. Therefore, they cannot violate this contract by raising your rates out of turn. If this kind of activity appears on your bill, then you should contact the company and have them explain to you what is going on. If they cannot, then you may need to seek legal advice to see what action you can take against the company if it comes to that.