The tax overhaul changed how much employers withhold from paychecks. If you withhold too much, you are due a refund. If you withhold too little, you owe. While the government urged people to review their withholdings to make sure they were up to date, few did. As a result, some people got more money in their checks during the year but were surprised at tax time.
The IRS urges all taxpayers to do a "Paycheck Checkup" now so that if a withholding adjustment is needed, there is time for it to happen more evenly this year. You can change your withholdings at any time.