Why is 1099-NEC reported as self-employment income? I went to work everyday, wasn't paying myself and waited for a paycheck, that's not self employment.

If you are receiving a Form 1099-NEC, you are being treated as an independent contractor by the company paying you and you need to file Schedule C for your self-employment income. 

You stated it yourself, 'I was hired as a 1099 contractor.'  That means that the company that hired you did not consider you their employee.  If they did, then you would have been issued a W-2 and not a 1099-NEC.  Therefore, you are responsible for paying the Social Security and Medicare tax in the form of self-employment tax.  That is why your tax return changed dramatically when you entered the income.  

The good news is that you can also claim expenses related to your work on Schedule C.  This will help to reduce your income tax and your self-employment tax.  

Take a look at the following TurboTax article for more information:

Am I considered self-employed?