Dear pattsy: There are many, many possible explanations for knee pain. Without evaluation, it is impossible to speculate on what might be causing your knee to ache. Our recommendation is to see your primary care doctor who can evaluate this pain and the possible explanations and then determine what additional evaluations may be necessary.
I'm not in the medical field but I have been breast cancer free for three years and recently found a bump on my shin bone which was also painful - it was quite upsetting. After three opinions that nothing was going on, I finally insisted on a full body bone scan and can put up with the diagnosis of "arthritis". I was also told that bone mets from breast cancer very rarely, if at all, will start in the extremities - more than likely would start at rib cage or clavicles. Bone scans are not fun to go through but peace of mind is worthwhile.