Getting erected with no stimulation, then calming down and erecting again without ejaculation.after few time testicles starts to hurts and swells. why?

Needs workup: Sounds like you should visit with a urologist. Not sure why you are getting erection although i don't think that's too uncommon for a 20 year old male. The swollen testicles is of more concern. You could have epididymitis. Epididymitis is the most frequent cause of acute onset scrotal pain in adults.

Normal: Just lots of testosterone, and subsequent frequent and easy arousal. See your doctor if erections typically last longer than an hour.