My left knee hurts on the right side (feels like on the inside) when i use stairs and move laterally. i don't remember hurting it?

Pinched cartilage: You might have pinched the inside part of the knee cartilage. It could have been hurt years ago, and re-injured by something minor. You could also have an inflammation of the tendon holding your knee cap down. If it is swollen or red, or doesn't get better in a few weeks, go see your doctor.

Inflammation: There are different reasons why you have this knee pain. Even though you don't remember an injury you might have irritated if you exercise. I would try a NSAID (ibuprofen, naproxen, etc) to see if you get any relief. Otherwise visit a doctor for an evaluation and possible x-ray and mri.