Have you taken the home tax credit? What year did you take it?
It seems that Uncle Sam is calling in the "debt" of the homebuyers tax credit...but only for 2008 recipients. The IRS says the 2008 tax credit was more of an "interest free loan", then subsequent program years. Beginning with the 2010 tax returns, recipients of the money in 2008 must begin repaying at $500 a tax year, up to their "loan" amount.
Interestingly, 2009 and 2010 "borrowers" do not have to pay anything back: the only requirement for these recipients is that they live in the property for 3 years. Meanwhile, the 2008 borrowers have 15 years to pay the money "lent" to them.
Not only did the government change the rules after 2008, they also increased the maximum amount from $7500 in 2008, to $8000 in the following years.