Lupus is often part of an overlapping connective tissue disease which can include a muscle disease like polymyositis or dermatomyositis. There are many other possibilities but it could be that lupus alone is causing your problems. More tests are needed to determine the cause of your weakness.
Is there any pattern to the weakness? Which muscles? Symmetric or asymmetric? Are there any other distinct symptoms?
When you see your doctor make sure you have written them out so you don't overlook anything.
I have a disease called mixed connective tissue disease which includes lupus, polymyositis and scleroderma. It can also involve other AI diseases. Like lupus, it is treatable.
Hope you get those tests as soon as possible. What type of doctor are you seeing?
MCTD (SLE Lupus, polymyositis, scleroderma). Diagnosed 2005. Kidney, liver, GI tract, dysphagia, raynauds, barretts esophagus, quadriplegic in 2005. Recovered and now active in downhill skiing, golf, hiking. Meds: amlodipine, benezapril, omeprazole, potassium, folic acid, vitamins, maxide.
Remain Optimistic and you can overcome.