Two days ago my index finger got extremely stiff out of the blue, swollen, red and looks bruised. i did not injure it in any way what could this be?

Over use: You are pointing that finger too often..... ( joking ) A simple cut (, puncture/twist may cause pain swelling.Paper). Keep clean, a finger splint,Aleve (naproxen) will help.

Many possibilities: Need to see the finger to understand if this is serious or not. From your description could be anything from a minor cut/ leading to low grade infection (pain/swelling) to acute arthritis (Gout), to a septic arthritis, or an advancing skin infection (often bacteria enter the finger from a small cut or at the nailbed). Finger infections can be severe, lead to tenosynovitis. Needs evaluation.