I have muscle pain, fatigue, stiffness, and occasional weakness that appears to be related to my autoimmune/inflammatory condition. And it definitely gets worse the more I use the muscles-- it takes me days to recover from things like even pretty gentle hiking. So, it's entirely possible that lupus could cause muscle inflammation (though I don't think that's common), or it could be another autoimmune condition affecting your muscles because, as we know, these AI things like to run in packs.
It sounds like your muscle pain and fatigue is severe enough and troubling enough to you that is sounds like you should definitely make an appointment with your doctor! I'd start with an appointment with your primary care doctor, who can evaluate whether this problem is likely related to your lupus, or something new. He or she will likely run bloodwork, probably for muscle enzymes that can show up in your blood when your muscles are affected by an inflammatory condition. Then he or she can refer you to the proper specialist, if necessary.
I really encourage you to see your doctor about this. Will you let us know how it goes if you do? I am always interested to hear how others deal with muscle problems.