Then the IRS should mail you a letter explaining it.
Did you make estimated payments and forget to enter them?
If you made estimated payments see if they are on line 18d. Sometimes they end up line 17 with withholding. If that is the case the IRS will eventually see line 17 is wrong and ask for it back.
And watch out for Scams. There is one that sends you a refund and then asks for it back. Be careful. Call the IRS at 1-800-829-1040 and make sure it's legit. Like this
https://www.irs.gov/newsroom/scam-alert-irs-urges-taxpayers-to-watch-out-for-erroneous-refunds-bewar...
