The picture is suggestive of stains ( mainly caused due to excessive intake of caffeine/tea/ coffee/ aerated drinks)
But since you complain of pain this is something to be looked it tooth.
The pain could be due to a cavity.
You need a dental visit
Next Steps
visit a nearby dentist asap. A proper consultation will help in better diagnosis.
Health Tips
Donot delay....
it may worsen and deepen the problem