What Causes Knee Pain After Shifting To Celebrex?

I have severe R.A. I had my meds changed from naproxen 500 mg. to Celebrex 200 mg. My problem is that once the new meds kicked in, I felt better. But from out of nowhere I have a severe pain in my left knee, it s not in the joint, but just below it. The thing is, it s been going on for a few day s now and getting worse. Plus it is also affecting my leg muscles. Do you think it could be from Celebrex? I don t want my Dr. to take me off if it s going to help me. I thank you for your answer.