Where on your tongue was the stain? Was it near you surgery site?
It's probably nothing,of course - but good you are seeing your doc.
My doc gave me a sinus rinse when one of my nostrils wouldn't stop dripping after my treatments.
I never did have to use it. My nose stop dripping the day he gave it to me. Perhaps my dripping nostril knew I was doing something about it...

But I've heard they are great. Much better than an enema. (hahahaha)

Have you ever used a sinus rinse?

Let us know what your doctor says.