IRS says we owe taxes from 2015 on $11000 as retirement income. We contributed to an IRA and filed 8606 in 2014. The $11000 was taxed as it was earned. Y taxed again?

In 2014, $11000 of our after taxed earnings was deposited as nondeductible contributions into an IRA. We received a letter from the IRS saying that we owe taxes on the $11000 Retirement Income from 2015. Why would we be taxed twice? Is this a mistake?