In the US, the recipient of a gift never pays tax or reports. The giver of a gift must report it if the amount is more than $15,000 per person per year, but only pays gift and estate tax once the total of their lifetime gifts exceeds about $11 million. (Similarly, your mother would have to make a gift tax return if she gifts the money back to you, whenever that happens.)
If this is a loan, the IRS will expect that the lender will charge interest, and you will be required to pay income tax on that interest when you would have received it. If you don't charge interest, the IRS will tax you on imputed interest -- the interest you could have charged if you were managing your affairs in a businesslike manner. The minimum imputed interest rate is variable and changes monthly but is around 1% APR. In other words, if your mother pays you back in a year, the IRS will expect you to report and pay income tax on about $3,000 of interest income even if you didn't charge your mother interest.
There is no particular trigger that would cause the IRS to audit you if you don't pay interest; taxes are on the honor system and most taxpayers are never audited. But, you should be aware of the requirement even if you decide not to follow it.
