If I forgot to report interest from bank, can I file a 1040X now to amend?

If your return has been accepted you can amend it. If the amount of additional interest is small you can opt for the IRS to catch it and send you a bill for the additional tax and a small amount of interest.  

Do not file an amended return until the IRS has processed your original return and you have received the refund.

How much is "a small amount"? There's no fixed rule, but if it's less than, say, $20, I would not bother to amend. The IRS will probably ignore it.