After you file

Yes, you need to amend your return to include the income, regardless of what else is going on with your return. Mail it return receipt requested, so you know when they received it.

It's great if they send you an official reporting form, so you should request one. They may have sent one to the IRS and that is what they are seeing as not being reported. In the meantime report it as cash and note no reporting form was sent. This is the second option below.