I Have Been Experiencing Slight Right Knee Discomfort. The Discomfort

I have been experiencing slight right knee discomfort. The discomfort is located on the outside of the knee. It started when I returned to the gym after a long lay off from the gym. I was walking fast on the treadmill and using a stair climber machine. Since the discomfort began I an limiting my cardio to the elliptical machine, low impact. It`s not gut wrenching pain, but a slight discomfort.