How much tax should be taken out of my paycheck?

I work at a hospital. I work nights and get paid hourly wages. I don’t make much. Depending on how much I work, I’ll make around 20–22k a year, before taxes. I get paid once every 2 weeks.

My first couple paychecks have been kinda strange. I have a little bit of state tax taken out, and some social security and whatnot, but the last couple times have NO federal tax taken out. My first check was over $400 (I worked 40 for one week) and only $30 was taken out for state tax and other taxes. I went over my W-2 with someone and it was correct. I’m really confused.

Since I’m moved out in, I can file as “Head of Household” because I’m single (not married) and contribute at least 50% toward expenses of the household. This is also my only job. I have no dependents, no children. I live with my boyfriend. So that’s how my W-2 was filled out and thus far, NO federal taxes have been withheld.

I did the IRS tax calculator and they said I’d owe something between 1–2k a year given my information, so WHY is no tax being withheld? I can’t figure it out. I’ve called work and they don’t know, especially since my W-2 was filled out correctly.

What do I do? I’m tempted to just put down nothing so they take out more than they need so I’d get a huge refund, but I have to live on this, so I can’t afford them taking tons out month after month.

I really hate paying taxes but I’d rather this stuff get deducted from my paycheck than owe the IRS a hefty bill at the end of the year. Noooo thank you.