How do you respond to "No?"?

You respond "No, I'm not sick".

I usually reply by repeating my answer and adding a brief explanation so that they know I didn't misunderstand their question.

For example:
"Are you sick"
"No."
"No?"
"No but my allergies have given me a nasty sinus headache."


Of course if you're annoyed they're asking you stuff that's none of their business and then questioning your answer, one might be tempted to reply with a little snark such as:
"Are you sick?"
"No."
"No?"
"Correct but I'm concerned there is something wrong with your hearing."

Of course your grindis lying. That's what text messages are for. Lying, or breaking up with you. If they weren't lying, they'd phone you.