Others have told you that muscle weakness can be a symptom of lupus. It can also be a symptom of a connecitve tissue disease like the one I have. Mixed connective tissue disease which is a combination of lupus, scleroderma and polymyositis.
Myositis (inflammatory muscle diseases) diseases like polymyositis and dermatomyositis will have symptoms of muscle weakness and swallowing difficulties. Getting up and down, climbing stairs, lifting overhead, and swallowing problems are common symptoms. PM hits your proximal (off your trunk) muscles more and DM will hit your distal (muscles farther from your trunk) muscles more. Skin rashes are typical with DM.
Steroids can also cause muscle weakness in some patients.
It is fairly common for lupers to have symptoms of more than one auto immune disease. If your muscle weakness is persistent and getting worse I would ask you doctor to test your CPK and SED rate. Unfortunately, myositis diseases are not always easy to diagnose and test results can be negative when you actually have the disease.....just like lupus.
I would advise you to push your doctor to find out why you are experiencing weakness. Good luck and hope you feel better.