What does it mean when my knee hurts when i bend but only sometimes when i walk?

Many causes: You could have a meniscal tear, a cartilage defect or fluid in the knee that makes the knee hurt. In older people, osteoarthritis is a cause. Sometimes, you can have a hamstring tendonitis or smaller ligament inury as well. See your sports medicine doc, primary care, or ortho if it gets worse or you've had it for awhile.

Patellofemoral : This is likely patellofemoral syndrome. It usually responds to physical therapy, activity modification and otc pain meds.