There can be many reasons for the teeth to discolor.
1) Stains can be external / extrinsic and include tobacco in any form, regular tea or coffee uptake or may be due to improper brushing or no brushing at all..
Treatment: such stains are present on the outside and can be removed by a procedure called scaling. Teeth can be polished after that and they appear good as original.
2) Stains can also be internal/ intrinsic. these can be due one of the following reasons:
a) from birth, transferred through genes, can effect enamel or dentin
b) Use of medicine like tetracycline antibiotics
when the teeth are forming.
c) Fluorosis : condition caused when there is more than required quantity of fluoride
in drinking water.
d) some diseases like porphyria
or neonatal jaundice
can also cause teeth discolouration.'
Treatment: intrinsic stains cannot be removed by scaling/cleaning. These also unfortunately cannot be treated by cosmetic teeth bleaching procedures. Most cases require percelain crowns or venneers as treatment procedures.
Most of the stains discussed above, usually seen either at an early age or involve whole of the dentition, both contradicting in your case.
Finally, looking at your details i assume that a single tooth has been discolored. This most often can happen after a trauma or if it becomes non vital (dead) due to any reason. A proper clinical examination supported by a X-Ray can diagnose it better. So please rush to a dentist
and have it checked.