My inner cheek and last corner gum has swollen. i can't open my mouth wide properly. it's paining and the inner cheek also has some kind of dark patch?

See doctor/dentist: This needs urgent medical or dental assessment. A dental infection that is compromising mouth opening needs to be rapidly excluded. There are other less serious possibilities such as periorofacial dermatitis, and other conditions that can cause pigmentation in the mouth, however if trouble opening mouth need to rapidly exclude serious causes. All the best.

Inflamation: Could be an injury to the area, from tooth brush, or an infection of some kind, like a herpes simplex. Gargle with dilute hydrogen peroxide for several days, and if not better visit a dentist or doctor for help.