Lower net pay is due to expiration of the payroll tax reduction
Two years ago, effective Jan. 1, 2011, the payroll (Social Security) tax was temporarily reduced by 2 percent. Although it stayed in effect a year longer than originally intended, that reduction came to an end on Dec. 31.
As a result, and all other things about your pay being equal, your net pay today will be at least 2 percent less than it was in December. Income tax rates were also revised, so you may see a change in your net pay due to that as well.