Pulled muscle maybe. Get back into shape with swimming.
Right now, we know exactly what's bothering you -- pain in the lower left back.
And as of now, we know one thing that can't be causing your pain -- kidney problems.
The rest remains to be found by a skilled diagnostician, through a thorough history, physical, and diagnostic tests.
It's your health, but if it were me, I wouldn't rely on guesswork.
Lower left back pain at the age you are may be muscle or tendon problems and also degeneration of the bones.
Muscle and tendon problems will take a while before they settle and massage and hot baths may help.
An xray or other imagining will confirm whether you have degeneration through wear and tear (osteoarthritis).
If you are still having the problem, you need to get back to your doctor for further tests.