I am male 19 year healthy but feel pain in my joints and lower back when i masturbate i use to masturbate in every 3 days ? reason for pain?

Unusual: I cannot see how masturbation and ejaculation could cause joint pains unless there is a positional explanation. Back discomfort could develop when the prostate gland is congested from too much sexual activity however. Are you sure that the joint pains don't occur on other occasions as well? Perhaps a consultation with a urologist may be helpful.