My ring finger feel stiff it's a bit swollen and when i bend it , it hurts. what is the cause of this?

If that finger is stuck at times when you hold something or during sleep, you have a condition called trigger finger (stenosing tenosynovitis of the proximal interphalangeal joint). If the other hand's finger proximal joints are also affected, rheumatoid arthritis needs to be ruled out. If the distal joint is affected, osteoarthritis is the cause but you are probably young for this.