What Causes Unusual Weight Gain In An Active Child?

About an hour after eating, my 14 y daughter completely crashes and even has a huge personality change. This is worse when exercise is involved. She can barely make it thru a tennis match without crashing. Her testing shows no significant decline in blood sugar to indicate hypoglycemia. Her pyruvic acid and lactic acid is consistently below normal every test and her alpha ketoglutaric acid is always extremely high. I feel that there is an enzyme or something that is not working properly but can't get the doctors to listen. She is incredibly active and cannot lose weight either. She does not eat junk food or drink colas yet she keeps gaining weight. Her thighs are disproportionately large. Her thyroid levels were fine.Could this be a metabolic/glycolic problem? Getting no answers from dr. here. I know something is wrong.