BBW,If you just noticed it, the black/dark red spot most likely represents an area of trauma, such as happens when biting your cheek. This could occur during chewing without you realizing it. It should disappear within a week, unless you keep biting it. It may also represent pigment of no consequence.
If that is the case then nothing needs to be done, but ruling out other worrisome diagnoses is warranted. Do your best to stay away from Dr. Google. I bite the inside of my cheek and rarely remember it until I notice the red spot. Remember we are not Drs. here so deciding when to see your Dr. must be your decision. Big breath and know you will be OK.
Kindly,Kitt