My Symptoms Started With Lightheadedness, Fatigue, Weakness, Tingling In The

My symptoms started with lightheadedness, fatigue, weakness, tingling in the hands, and feet. I was diagnosed as being deficient in vitamin b12 and D. Have been on the prescribed supplements for 2 months. The fatigue, weakness and lightheadedness never went and in fact seem to be worsening. I have been to a hemotologist, a GI doctor, endocrinologist, and am currently undergoing testing by a cardiologist. All of the bloodwork done by each Doctor looks great and the b12 is within normal range. My blood pressure is low normal range but when I go from sitting to standing my blood pressure increases slightly but my heart rate will jump from 57 to 82. I am not dehydrated. I am at a loss. If the cardiologist cannot make a diagnosis my next stop is a neurologist. Like I have stated before, it seems as though things are deteriorating. I am a 40 year old female, mother of two young children. Any ideas of what could be going on or anything that would help my current Doctor?