Constant knee pain in left knee, no injury, hard to walk, pain behind knee cap, knee gives out what can cause this?

Cartilage/ligament: This could possibly be a either a meniscal (cartilage) injury or a torn ligament. Any time one describes a knee "giving out" it sounds like a possible ligament injury, although with a ligament injury most likely you would have known there was an injury. A meniscal tear can be tricky and a possibility. Any time knee pain more than 2-3 weeks despite rest and not improving consider ortho visit.

See a specialist.: See an orthopedic knee specialist. You may have "patellar pain" or a variation of it, based on what your describing. If so, it's usually treated with medication and physical therapy.