knee-leg pain

I'm not sure what's wrong and since I can't get in to see the Doctor till next week, I hope you can help. A few weks ago I began experiencing pain in my right knee that I put off to arthritis-I'm a 47 yr. old female with no medical problems, avg. weight and health. I've experienced mild arthritis for the past coulple of years, but this is different. The pain is continuous and is not helped by elevation or heat. It radiates from behind and on the rt. side of my right knee and extends down towards my foot. The pain throbs and seems worse when I lay down. Standing and walking are less painful than inactivity. Bending hurts a little more. Ibuprophen and asperine offer mild relief. What the heck is wrong? I can recall NO strenuous activity prior to the onset of pain. Am I just getting old?