Please visit an oral medicine or oral surgeon for proper examination. Though it doesn't seem to be anything suspicious and could be a cheek bite with clotted blood. But fro exact diagnosis you need to visit a dentist.
Next Steps
You can also observe it over for a few days and if the size doesn't increase/ or the lesion changes color towards normalcy/ or it disappears, there's nothing to be done. However, if the lesion increases in size immediately seek medical attention