Oral-B Pro Expert Toothbrush Caused Mouth Sores — Requesting Refund

I purchased the Oral-B Pro Expert toothbrush from a supermarket about a week ago. From the very first day I started using it, I developed sores in my mouth after each use. This has been happening for a week straight, and I’ve had to stop using the product because of it.

I still have the product packaging. I am requesting both a detailed explanation from the brand regarding this issue and a refund for the product.