I was in a similar situation last year.
Your subsidy is calculated based on several numbers:
1. Your family size.
2. Your family's adjusted gross income.
3. Your location.
4. The second lowest cost silver plan in your area (aka the SLCSP).
(As an aside, your subsidy is *not* based on the insurance plan you choose, as long as you're choosing among the plans on the exchange.)
What has likely changed in your situation is item #4. You can get an idea of what happened either by (a) talking with your state exchange people about silver plans in your state last year vs. this year, or (b) digging out your tax return from this year and looking at the SLCSP premium reported on your 1095 or on your Form 8962 and then going into shop for plans for next year, restrict the view to silver plans only, then sort by price and look at the second lowest cost plan.
What you're going to find out is the the SLCSP has changed in price by about $250 per month, which results in your subsidy changing by the same amount if everything else has stayed the same.
...
I suppose there is an outside chance that your state exchange has messed up your information. You might want to call and confirm that they have the correct family size, income, and address. My state went in and deleted two of my dependents this year, which I had to call in and have them reinstate.
