My knees always have crackled and pop when i bend them but now my left knee hurts really painful to bend down and come back up. what could this be?

Chondromalacia ...: Sounds like chondromalacia patelli or patello-femoral pain. That is an inflammation of the jint between the kneecap and the underlying groove in the femur. More common in women than men due to the mechanics of the knee and wider pelvis in females. Classically painful going down stairs and getting up from a chair.

Could be instabilit: Could be knee instability, internal problem or arthritis.

See your orthopedist: Sounds like you may have a meniscal tear or cartilage injury.. Your symptoms are typical - including pain and locking. Other symptoms may include swelling, clicking, and instability. Check in with a local orthopedic surgeon for an evaluation. She will probably recommend physical therapy initially and perhaps an MRI depending on you examination. Arthroscopic surgery may be necessary.