I have a tiny red line on one of my back teeth on the side facing my tongue. i am wondering what it could be. it doesn't hurt or bleed. it's pretty small and doesn't surround the entire tooth. at first glance it looks like the gum line?

I doubt that what you have noticed in your mouth is anything to worry about. You should check this area once a month and see if it changes in color or size. Unless it grown in size or becomes black or more brown, you can ignore it as normal(?gum line).

We cannot diagnose on this site. We do not have advantage of dental history, examination, imaging, pulp test, etc. If you are concerned, see your Dentist, especially if "line" changes over time.