Here’s a professional and concise draft you can use to inform your office:

Subject: Sick Leave Request

Dear [Manager’s Name],

I am experiencing severe gastroenteritis symptoms, possibly due to food poisoning, since Saturday. As my condition is worsening, I will be consulting a doctor today. I am currently unsure how many days I will need to recover but will keep you updated on my progress.

I will submit a medical certificate as soon as possible. Thank you for your understanding.

Best regards,
[Your Name]

This conveys your situation clearly and professionally.