I Have Had Severe Pain In My Metacarpophal Joint (middle

I have had severe pain in my metacarpophal joint (middle finger knuckle). It is going on for a month and not getting better. It hurts to extend my middle finger and when I close it or make a fist it locks closed. I am losing grip, difficult to write, play musical instruments and when I make a golf swing it is painful to open my finger. Sometimes I have to use my left hand to unlock my finger. No other fingers are affected. It started suddenly. One night while sleeping I was laying my head on my hand in a fist for a long time. When I awoke I discovered my middle finger locked closed.