Celestine if you’re going to treatments, this is a side effect and you need to communicate this with your oncologist.
When I was going through chemo treatments, I was prescribed Valacyclovir to prevent this from happening.
I had RCHOP 21 chemo.
It’s also advisable to use a very soft toothbrush and to rinse your mouth out with salt water. Rinsed my mouth out with water. Every time I would use the restroom this helped.

Thank you for your reply. I will try the salt water rinses