I am a 21 year old female, 5 4, 135lbs. Recently, i have been experiencing some pain in my left knee. Every time I extend it, i feel a popping sensation on the outside of my knee cap. It does not hurt to put weight on it, for i experience pain only when i am mobile. This has been going on for the past couple of days, but it seems to be getting progressively worse. Its beginning to hurt to walk up stairs. I have not been doing any strenuous work lately to cause such a painful injury, but I have hyperextended my knee before, several years ago. Please help, as this pain is beginning to become excruciating. Thank you.