Every year, about 100 million Americans receive tax refunds from the federal government. Last year, the average refund was $2,800 and taxpayers used it for everything from paying down loans to putting down payments on cars to taking late-winter vacations.

This year, experts agree, things will be different. You tax refund may come later than you expected, especially if you file early. It may be lower than you're used to. Or higher than you're used to. Or, you may find that even though you've always gotten a tax refund in the past, this year you owe taxes to the federal government. No one seems to know for sure.

What the heck is going on? Here's an explanation, or at least as much of one as we currently have:

1. Why your tax refund may be delayed.

You know the answer to this one already. The federal government shut down for 35 days and may shut down again on or after February 15 if Republicans and Democrats in Congress and President Donald Trump can't agree on a spending plan for border security. The IRS has not--so far--said that refunds will be delayed. But it's a pretty good bet that they will. First of all, the agency has to dig out from under a lot of backlog. During the shutdown, it was operating with only 12 percent of its usual staff. That meant, among other things, that people couldn't easily call the IRS so instead many of them wrote letters. The result is that the agency now has 5 million pieces of mail awaiting response. By some estimates, it may be as much as 18 months before the agency is caught up on all its back work (although no one is suggesting that refunds will be held up as long as that). Of course, if there's a second shutdown, things could get worse.

2. Why your tax refund may be lower than expected.

The rest of the uncertainty around your tax refund results from the tax reform bill passed in late 2017. Chances are that the new law will reduce your taxes, at least until 2025 when some of its provisions phase out. That means both you and your business are probably paying lower taxes on your 2018 income than you did on your 2017 income. 

But the fact that you owe less taxes for the year does not necessarily mean you'll get a bigger refund, or any refund at all. Your refund may feel like money that falls from the sky, but it's really just your own money that you overpaid for taxes coming back to you. In order to get a refund, you have to have overpaid, either in taxes withheld from your paycheck or in quarterly estimated taxes if you paid those.

Did you overpay? Maybe, or maybe not. Accountants and accounting departments have tried to adjust to the new tax law by reducing the amount that is withheld from people's paychecks or that they pay in quarterly estimated payments. If you paid lower taxes throughout 2018, you may already have received all the benefit you're going to get from the new tax law, and you could have less of a refund coming. Depending on other factors, you might even owe money (see below). 

You can find a clue to the likely amount of your refund if you compare a 2018 pay stub (or estimated payment) to 2017. If the amount of federal tax withheld from your pay, or paid in estimated tax, was a lower percentage of your overall income than the year before, you could have a smaller refund coming, or even none at all.

3. Why your tax refund could be higher than expected.

By the same calculus, if the federal taxes withheld from your paycheck, or the estimated tax payments you sent to the feds, are a similar percentage of your overall income as last year, then there's a decent chance your refund could be bigger, since you may owe less in federal taxes overall. However, a few commonly used deductions of past years were reduced by the new law, which means depending on your circumstances you could end up owing more.

4. Why you could end up owing taxes.

Tax reform lowered income taxes overall, but it didn't lower them for everybody. That's because a few juicy deductions were reduced in the new law. Most states and some municipalities collect income tax, and up until 2017, all of that state and local tax could be deducted from your income when calculating federal taxes. Beginning in 2018, state and local tax deductions (for both income tax and real estate tax) are capped at a total of $10,000. If you live in a state and/or city with high income or real estate taxes, this could mean you owe more federal tax under the new law instead of less. If you failed to increase the taxes withheld from your paycheck, or failed to raise your estimated payments to account for these higher taxes, you may find you owe money to the IRS instead of the other way around.

Also, if you own your home and it has a mortgage, you're probably used to deducting all the interest you paid over the course of the year from your income. You still can--if your mortgage is lower than $750,000 or if it originated before December 15, 2017. But if your mortgage is higher than $750,000 and you took it out since December 15, 2017, then you can only deduct interest on the first $750,000 of that loan. You should consider this new law and its effect before refinancing a large mortgage that dates to before December 2017. In any case, if you have a large mortgage on a home you bought within the last year, your interest deduction is lower than you might expect. Depending how much you have already paid in taxes withheld from your paycheck or estimated payments, you could owe more. 

Many of us have home equity loans or lines of credit (commonly called a HELOC). If you took out your home equity loan to make improvements to your home, then it's unaffected by the new tax law. But if you used that money for any other purpose, your home equity loan's interest is no longer deductible at all. If that's the case, you could owe more taxes than you expect, and you could wind up owing money to the IRS rather than getting a refund.

So there you have it. Your tax deduction this year could be lower than you expect, higher than you expect, or none at all. Whichever it is, it will probably be late. So the best advice is probably this: Don't plan that vacation or that shopping spree until your refund money is safely in the bank.