A polite response to someone asking why you are not at work or school could be:
"I appreciate your concern. I'm currently taking a personal or sick day today, but I'll be back at work/school tomorrow. Thank you for asking."
Keep in mind that the level of detail you provide in your response is entirely up to you. You are not obligated to share personal information if you don't feel comfortable doing so. It's essential to set boundaries and respect your privacy. The example response above acknowledges the inquiry without going into specific reasons.