Tax form

I’m not sure where to ask or if this is even the place but I need some help as I’m a young mom trying to figure things out on my own and don’t hVe a lot of help… I got my w-2 back and my job has not been withholding federal taxes BUT on the last month of the year in dec they did for one check, 166 dollars.. why did they do this? Is this a mistake I need to ask them about?