After you file

Look at your Form 1040. Is there an amount on line 21?
<a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-pdf/f1040.pdf">https://www.irs.gov/pub/irs-pdf/f1040.pdf</a>
If so, only the IRS can tell you why they sent you the money.