Why sometimes when i take deep breathes i feel a pain in my left knee???

An increase in intrabdominal pressure by expanding the lungs could lead to an increase in intra-spinal pressure. So could a sneeze or bearing during a bowel movement. If there was a narrowing of the nerve foramen for the L4 nerve root (the nerve supplying sensation to the knee) or a bulging disc at the region of the L4 nerve, the aggravation from the increased pressure could cause your symptoms.