I just found a small black/ dark red spot inside my cheek above my back lower molars. What could this be?

Trauma. 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 you use dip or chew, it could be serious & you should be checked by your doctor or dentist.