I amended my return via TT to add long term capital gains income. I filed it via TT as soon as it was available (Feb 23rd, I believe) . I made full payment for the tax owed at that time. At the beginning of April, the IRS sent the full amount in a return check. I was advised not to cash the check, as they might charge me interest when the error is detected. How should I handle this?
You can't return the check. Deposit it into your bank account but don't spend the money. You should be prepared to give it back to them if they ask.
You should have received a letter from the IRS explaining the changes made to your tax return. If you haven't received it yet then be patient. It can sometimes take a year or more.
You can also log in to your account on the IRS website and see if there is an explanation there. Maybe they lost the amended return? Maybe they found a mistake that you made that was in your favor? You can check the account online and see what it says.
Here's where you log in or create your account.