I have pain in my left sacroiliac joint while sitting or standing since 3 months. i am having numbness below knee joint(left) since 3 weeks ?

Numbness and pain: Pain in the si joint when combined with numbness below the knee is very worrisome for a disc herniation of the lumbar spine. Anti inflammatories and proper exercise are very important. If your numbness and pain are significant i would seek out your orthopedist. They can offer you more help and insight for your problem.

Lumber disc problem : Since you have in lower back and numbness in the leg, the most logical answer is disc problem in the lumber spine . Check it out.