Is Unitedhealthcare Committing Fraud?

AARP recommended UHC for Medicare Rx coverage so I got their plan. Last month I received a bill that was almost double what the price was in December! I spoke with their customer service folks and was told that none of their customers were notified of the premium increase and that they were within the Medicare laws to do so. Interesting how this happened a month after Medicare Open Enrollment closed so now I am stuck paying higher premiums for 10 months until it opens again. I filed a claim with my State insurance commissioner who kicked it to Federal because it involved Medicare, so naturally have not heard back. Regardless of integrity issues, is this bullshit even legal?