Knee problem

I need help in figuring out if I should see a doctor…I have pain in my left knee that feels like it is predominantly in the joint (outside of knee – potentially runners knee or IT band but not certain as this is very painful). I am a runner -so not running has been incredibly difficult for me and I have been trying other forms of cross training. It started out as a minor irritation when running after 3 miles…now is to a point where I cannot bike without alot of pain in the joint…The only tell-tale event that brought this on was 1week after I ran a half-marathon, I was walking up a hill and felt a small pain in the outside joint are of the knee. It has been plaguing me since and has been only getting worse…Are there certain tests I can somewhat apply to myself (I noticed a few already on the generic Knee Injury page)to help me determine what might be the problem? I like to be knowledgeable before I see a doctor (which I cannot stand the thought of but know it is getting to that point)…If you need background info on my injury I will respond. Thank you!