I'm not sure about arthritis affecting both and not a single limb joint. I have had arthritis in left knee for over a decade due to an old basketball injury & not my right. Same with my ankles & even my carpal tunnel is more progressed in my predominant hand. I've found even legs are predominant-like climbing steps, I will naturally use one more than the other (or like climbing a ladder) perhaps you've strained one side more than the other/has taken more wear. Also having several recent knee injuries, X-rays don't tell much. I had multiple "normal" X-rays but it was after several trips to my ortho (sorry urgent care centers are designed for intricate diagnostic purposes) that he ordered an MRI that showed several ligament tears & degeneration. Bone scans are also helpful in accessing deterioration of joints & bone. Sciatica can also cause phantom like pain in knees, hips & ankles-pain so severe that'd you'd swear the painful area is indeed a separate incident. Keep in mind that nerves-especially in your legs-are directly effected from lower back pain. Have you ever gotten that MRI done of your spine yet? If you do have an lupus diagnosis it might be worth bringing up to your rhuematologist as well-mine has informed me that lupus makes me more susceptible to ligament & tendon tears. Keep on pushing, hope they can provide answers & the relief you need!