Pain When Eat & Black Line On Tooth - Worried- Any Advic

Hi (im very worried any advice)

i'm a 21 year old female and I have had pain in my tooth for the last week now.
Its a back tooth. I dont have any wisdom teeth,
its not my very back tooth, nor the one next too that, but the 3rd from the back- at the bottem.

It only hurts when I eat but not everytime, I think hot tea brings the pain out.
I wouldn't say its bad pain. But I notice it when I eat something and it touches the top tooth. But when I bite down without food in my mouth I cannot really feel any pain.

There is a very dark brown line on the tooth and a small dot, i'm 21 and it's been there since I was about 11years old.

I have been to the dentist every 6 months my whole life and shes never mentioned this mark. I haven't because she obviously hasn't said anything, so it must be harmless.

Any idea too what this might be, would be great.

-the gum around the tooth is normal.
-and the tooth colour is normal.
-brushing doesn't hurt it.
And I had a cold and as the cold went it came, but maybe this was just a coincidence!

That's about all I can think of to say. I hope this helps someone to perhaps no what it might be.

Hope someone can help.
Regards,
kate.