You are not self-employed. Your income from selling cryptocurrency is investment income, not self-employment income. You do not pay Social Security or Medicare taxes on investment income.
If your gross income is over $200,000 (filing single) you might have to pay 3.8% Net Investment Income Tax on part or all of your investment income. Some people mistakenly call this a Medicare surtax, but it is not for Medicare.
Does "a -224 Captial gain" mean you have a $224 loss overall? If so, there should be no need to make quarterly estimated tax payments, unless you did not have enough income tax withheld from your pay when you were working. If you have a significant profit on the cryptocurrency sales, you would have to make estimated tax payments. But that seems unlikely if you always sell the cryptocurrency as soon as you get it. You still have to report the sales on your tax return, though.
