hello and welcome to HCM forum,
according to the history provided by you,it seems some kind of pigmentation spot has appeared which is nothing to worry about,
this kind of pigmentation appears due to number of factors, like vitamin deficiencies, oral medication, etc.
i would recommend maintain your oral hygiene
as you are,
use an antibacterial mouth wash(twice/day)
you gums with your finger for atleast a minute,
take oral antioxidant capsules
for a month(1capsule/day),
i wish you good health.