It all starts to make sense now.

My knees and shoulder joint (left) hurts when I exercise.  It does not hurt if I do not exercise.

Looking back, I was cutting down on meat intake and increased tofu/mushroom/beans intake.  I believe this is the culprit.

My diet is not giving me enough protein to repair muscle and maintain muscle mass.  So my muscles are weak.  When muscles are weak, they are not able to protect the joints during exercise, resulting joints injury!
 

Dr. Sanjay Gupta (CNN medical correspondent) also says that muscle mass loss increase the risk of getting dimentia/Alzchimer.

I need to take more lean meat/fish.