I was stretching for lotus position in yoga and i heard a pop in my left knee and now it hurts on the outside of my left knee?

See below: This is a problem with this particular pose which can put a lot of stress on the ligaments of the knee. Many people (including myself) cannot do a full lotus pose; their hips are not open enough which ends up putting a lot of stress on the knees. You may have torn your lateral collateral ligament. See an orthopedist or a physiatrist for evaluation.