Someone bumps into me when i was running yesterday, my lower back hurts a lot after that. could it be my kidney was damaged?

No: Usually requires a fairly significant blunt force to the flank or mid back region to cause an injury to the kidney such as might occur during a severe fall, car accident, or hard contact sports. Significant kidney injury often associated with outward skin bruising overlying the kidneys and blood in the urine. Most likely, you sustained a sprain or strain to your back muscles which can be painful.