What Does Pain In The Right Knee Indicate?

hi, i am a 68 year old female. I developed this pain around my right knee about a month ago, there is no swelling or redness and it does not feel hot. I have been limping on that leg. I have been using a knee brace and taking Motrin for pain with no relieve. I went to see my PCP and had an x-ray of the knee done last week. The results were negative, no fractures or broken bones. I was told that it is arthritis and keep doing what i am doing , just add glucosamine. Right now there is no change, i am still in pain and limping.The pain feels like an 8 on a scale of 10 and it feels like it is in my femoral bone. It is keeping me up at night and preventing me from walking any long distance. My question is... could that be symptoms of sarcoma ? what should i ask my PCP to do ? Thank you.