You were treated as a contractor for your side job, not as an employee. When you're paid this way, no taxes are withheld from your earnings and they are reported on a 1099-MISC (instead of a W-2). As a result, the IRS treats you as self-employed, and you'll need to pay both income tax and self-employment tax (Social Security and Medicare taxes).
Please see the FAQs below for more information.
Does a 1099-MISC mean I'm self-employed? https://ttlc.intuit.com/replies/4770969
