1099-k GrpupOn income difference

I just received a note from the IRS stating that GroupOn reported $44,000 on their 1099-k form, but I only claimed $33,000 because that is what I received in my bank account. Now the IRS wants to tax me on that $11,000 difference. But is that correct? Or do I need to contest this alteration and send them my 1099k and bank statements?