The Affordable Health Care states that the Premium Tax Credit is based on your annual income (all of 2016). If you weren't making enough money for part of the year, you received help from the government to help pay for your health insurance. Once you changed jobs and were making more money, the government says you can now afford to pay some of the money back, that they gave you for those months you needed help.
When you take the Premium Tax Credit as a monthly subsidy, the credit acts like a discount and reduces what you pay monthly for coverage.
However, if your family’s situation changes during the year, the premium tax credit you qualify for also changes. If you forget to update your original estimates, you may need to pay back some or all of your credit on your tax return.