I would suggest that you have this issue checked out by your primary care doctor
, especially if the symptoms persist or worsen. They can examine for any signs of trauma, infection, or swelling in your mouth and throat and help you determine if any sort of treatment is needed.
One possible explanation that comes to mind is that this black spot may represent a blood blister. Frequently, the lining of the inside of the cheek is subject to minor trauma, for example from accidentally biting the area with your teeth. Most of the time, when we do this we don't have any recollection of it, as it may occur when we are eating or when we are sleeping. Generally speaking, if the issue is minor trauma, then the pain and swelling will gradually improve over a few days and the blood blister collection will be eventually digested and cleared away by your body's normal healing mechanisms.
Another less likely but more worrisome possibility is that the black spot is a melanoma, which is a pigmented form of skin cancer which can sometimes occur inside the mouth.
Start by seeing your doctor at your earliest convenience, as they should be able to help figure this out for you.