I think they adjusted your refund for something else. So you had the 1600 on 1040 line 26? Do you have proof you made the estimates? Bank or credit card statements? Maybe they got applied to the wrong person's IRS account. Maybe you entered them twice? Is line 26 right?
You can check your IRS account. How to view your account at the IRS and see your payment history and payoff amount, etc.
https://www.irs.gov/uac/view-your-tax-account
The IRS will mail you a letter if they adjusted your return.
