Thank you for asking. I do understand your concern. It could be a problem of premature ejaculation. The threshold for ejaculation in such a condition is very low,  and minor arousal or excitement often leads to discharge. There can not be a specific cause to it but many factors. Stress and anxiety are two major risk factors for premature ejaculation. With suitable interventions in lifestyle and if required, with medication, the problem can  be completely cured. It doesn't have any effect on kidneys. Kidneys and masturbation are not related. Although excessive masturbation can lead to sprain of abdominal muscles, that can cause pain in abdomen.