It is common for lupus to attack your joints and many, if not most, of us have that to some extent. In my case lupus hit all my joints but once it was under control I have only had sporadic problems and at my age it would be difficult to tell if it is now just osteoarthritis or lupus.
Lupus also can attack your kidneys and cause edema typically in your feet, ankles, and legs. I also had that and had edema in my arms, hands, wrists as well. Gained as much as 20 lbs of water weight until the doctors treated the kidney condition which is called proteinuria. I looked like the proverbial Michelin man...very round and swollen. When you have that much swelling in your joints they are difficult to bend.
How are you being treated for lupus? Have you seen a kidney specialist (nephrologist) to make sure your kidneys aren't involved? If they are he will treat them and your swelling should go away.
See your doctor soon.