No, I don't think you're stupid. You are thoughtful. As you said, she spent a lot of $$ on that machine, and you would feel horrid if she just sent it to you without first insuring it was a machine she really didn't want/need. But I would call her back and just work into the conversation that you'd be happy to take it off her hands and get it out of her way if she is ABSOLUTELY CERTAIN she doesn't want it. You may even offer to pay her something for it (perhaps offer to quilt all her tops at no charge in return for the gift?).
I agree it is an incredibly generous offer, but you also don't want to appear to be taking advantage of grannie's better nature.