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This is Dr .David
you could have bit the inside of your mouth last night and caused a bruise which can look black on the inside of your mouth
or you could have something like lichen plantus on the inside of your mouth.
is the area sore?
do you grind your teeth at night time? many people do and don't realize it when they sleep.
it could be lichen plantus.
I can't see or examine the area.
you might need to see a dentist or oral surgeon and get a biopsy of that area for diagnosis.