federal witholding is not being taken out of my pay

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alright I got my second paycheck tonight after I got my job back and I noticed that social security, city, and state are enjoying my money but the federal government is not taking anything out of my check, what does this mean? am I going to owe the government hundreds of dollars at the end of the year? I thought it was just because it was my first check the first time but 2 checks in a row has me worried, should I start saving up some money just for the purpose of paying a huge debt?