You are a business in the IRS eyes.
You do not have to carry on regular full-time business activities to be considered self-employed. You will notice the amount you were paid probably appears in box 7 of your 1099-MISC. This is non-employee compensation. You were contracted to do a job, not employed to do a job. You will need to file a Schedule C for Business Income and Expenses and pay Self-Employment tax on this income.
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