As @xmasbaby0 implies, your situation strongly suggests that you did not report your self-employment income on Schedule C of your tax returns as you should have. If you had done so, TurboTax would have automatically generated Schedule SE, which calculates your Medicare and Social Security tax.
If your previous year tax returns do not include Schedules C and SE, then you can file amended returns.
IRS info on amended returns:
https://www.irs.gov/taxtopics/tc308
Schedule C:
https://www.irs.gov/pub/irs-pdf/f1040sc.pdf
Schedule SE:
https://www.irs.gov/pub/irs-pdf/f1040sse.pdf
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
