$9,000 would probably be about right if your were single, didn't itemize and all your income was from self employment - $4000 in regular income taxes and $5,000 in self-employment tax.
one way I can think of you owing an additional $5- $10,000, if the IRS thinks you failed to report all your income. This could be just a pure mistake on their part or someone filed a income form with your social security number. before calling the IRS you may want to go to its website and get a transcript for 2018 to see what the IRS changed from what you filed.
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