thanks for writing to HCM,
As per your description your pain is due to a blunt impact on your back which is painful and also radiating to your leg.
In this situation we need to see the site of impact of your lower back area for any local swelling, change in skin colour, any bruising. If all of this is present then it is only a localised injury of your back muscles.
Also since the pain is radiating into your leg the chances of back muscles spasms and inflammation are more.
Chances of damage to your kidney
are less but if you really want to be sure of it then an ultrasonography
of whole abdomen will help you in doing that as it shows kidneys clearly.
If the pain is due to previous reason of muscle injury(soft tissue injury
) then localised hot water fomentation & rest should help you.
Or you can take the help of your orthopedic specialist in this case.
I hope this helps
wish you good health