If you went $7k in to the 25% bracket you paid $1,750 in taxes that could have been avoided by diverting that to a Traditional 401k. It's really that simple.
If your tax due amount comes out to $5,000 more than the prior year but your Adjusted Gross Income (bottom of Pg.1 and top of Pg. 2 on Form 1040) didn't change much, then you screwed something up.
Check the rest of your form, compare it closely to the prior year. Did you claim your personal exemption? How about your deductions? Did you have the standard deduction in both years? Itemized one year and standard in the other? Did you miss the standard deduction all together? Something doesn't sound right.
on edit:
No, you can't go back on 401k contributions and change them at this point. If you want to drop your taxable income some and you haven't made any IRA contributions you can make a Traditional IRA contribution of $5,500 to drop your income by that amount, it would reduce your tax bill by $1,375.00