Yes you had zero income after the Standard Deduction so you probably had zero regular income tax on 1040 line 16. But if you showed a profit on Schedule C you owe Self Employment tax on the profit. The SE tax is separate from income tax. Self employment tax is on 1040 line 23.
You will pay Self Employment tax (Scheduled SE) on a Net Profit of $400 or more on Schedule C in addition to regular income tax on it. You pay 15.3% SE tax on 92.35% of your Net Profit (If it is greater than $400). The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare. So you get social security credit for it when you retire.
