hello and welcome to HCM forum,
i would like to inform you that discoloration of the tooth is either due to tooth decay
or due to staining.
i cannot check your tooth clinically, therefore according to the dental history presented by you it seems to be due to dental caries(decay).
To avoid the decay from spreading ,i advise you to see a dentist
and get the tooth filled.
your dentist will remove the decayed portion and place a restoration (filling) on your tooth, it is a painless procedure therefore do not worry.
i hope i answered your query,
i wish you good health,