Suggest Treatment For Fatigue And Muscle Weakness

My complement C3 has been tested twice since February and both times it has been 41 mg/dL which is far below normal range. My complement C4 was slightly low one time but normal low range the most recent time. CH50 was also low normal range. A rheumatologist I saw is seeing no rheumatological disease to explain but everything I have read says it is important to determine if the deficiency is genetic or an aquired one. For the past 2 years I have went from healthy and active to being severely disabled by fatigue, malaise, post exertional malaise, muscle weakness, neuropathy and so on. I am a thin, 38 year old woman. Doctors were thinking Systemic Exertion Intolerance Disease, but low complement is not at all associated with SEID. Also my Natural Killer Cell Function is much below normal range. What kind of doctor can help figure out why my C3 is so low? What kinds of tests should be done to determine cause of it?