Medicare premiums can change year to year based on your reported income.
If your adjusted gross income (AGI) plus tax exempt interest is over $85,000 for single taxpayers or $170,000 for married couples, you will be subject to the income related monthly adjustment amount (IRMAA) in addition to your regular Part B and Part D premiums.
To determine your Medicare premium the Centers for Medicare and Medicaid Services looks at your tax return from 2 years ago. So for 2016 they will base your premium on 2014 tax return. The new rates are announced in October for the following year. This table shows premiums for Part B & D for 2015 based on varying income levels.