While it might lead to pretending to be well some of the time, I think you might get milage out of, "Okay, you're sick? Time to go to bed, no candy. No, you said you were sick. This WILL HAPPEN NOW. No take-backs. If you weren't really sick, you shouldn't have said it."

And then stick to it. Don't let her back out of it. (Hard on you, I know. But "takebacks" on being sick are longer-term annoying. We had little miss takeback for a while. "I want X! no, Y! no, X!")

It might give a few false negatives, when she feels a bit ugh and doesn't want to miss something, but it should eliminate the false positives after she realizes that you're not going to let her take it back.