It is difficult to answer precisely without seeing your tax return. I can think of several reasons for this increase:
1. Adding your interest could have pushed you to the next tax bracket which is 24%.
2. You may have to pay the Net Investment Income Tax (NIIT) of 3.8% on the interest/ See this IRS document.
3. Adding to your AGI may reduce your earned income tax credit.
The Earned Income Tax credit (EITC) is a bell shaped curve against income. The EITC rises with income until a certain level of income when it starts to decrease to reach 0 at the maximum level of income for the category.
As an example, please look at the EITC tables from page 27 of this IRS publication.
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