Other possibilities for your symptoms are neuropathic postural tachycardial syndrome(also called hyperadrenergic orthostatic tachycardia, or postural orthostatic tachycardia (POTS), dehydration,hypoglycemia,anemia,
eating disorders and cardiac and neurological causes.
Pls discuss these options with your doctor so that the cause of your tachycardia can be found out.
Hope it helps.Take care and pls do keep me posted in case of any additional queries and how you are doing.