Self-employment tax, probably.
SE tax is the equivalent of Social Security and Medicare tax. You pay this even if your (taxable) income is low or even $0.
SE tax is separate from regular income tax, and is calculated on the net income from self-employment (Schedule C/ Form 1040, line 12).
You can owe SE tax even if you owe no regular income tax due to having no taxable income. Taxable income is a calculation on your 1040, and equates to your total income, less your exemptions and standard/ itemized deduction.
For instance, you can have net Schedule C income of $7000. If single, your standard deduction and personal exemption would subtract $10,350 from that. So, your taxable income would be $0. You would still owe SE tax on the $7000 net income from self-employment.
You can preview your 1040 here:
- My Account
- Tools
- View Tax Summary
- Preview 1040
- Look at lines 57 to 61... Is the tax due coming from one of these lines?