There are many factors involved, but it doesn't seem that out of range because you owe not only ordinary income tax on the additional $17K but Self-Employment tax also. The Self-Employment tax is an additional 15.3% of your net profit. You pay ordinary income tax plus the employee and employer's share of social security and medicare taxes.
Adding your $14K of self-employment income last year cost you about $4K in taxes, so it stands to reason that adding $31K of self-employment this year will cost you more than twice what it did last year in taxes. Make sure that you are deducting all of your business related expenses; this will help lower your net profit and the amount you pay SE tax on. See the second link below for some possible tax write-offs for your self-employment income.
To see how your taxes were calculated, look at your 1040 and your Schedule SE. See the links below for more information on the Self-Employment tax and how it affects your personal taxes.
https://turbotax.intuit.com/tax-tools/tax-tips/Self-Employment-Taxes/The-Self-Employment-Tax/INF1202...
https://turbotax.intuit.com/tax-tools/tax-tips/Self-Employment-Taxes/Top-Tax-Write-offs-for-the-Self...
