possible blood sugar issue?

I have a friend who's 6yr. old has been having sudden onset severe mood swings that nothing will calm her down and then she immediately eats her food like she has never eaten before and then is a totally different person. We're talking sudden break down with screaming and crying, raging and mean, with no relief and nothing will console her.  We give her something to eat and suddenly it was like it never happened.  These mood swings don't happen very often, sometimes she can go months without them.  But what has us concerned is the way they come on so sudden and are relieved with food.  This is a girl who hates every food put in front of her, but she will eat, it just may take her an hour.  There is no lack of food or snack if she asks when she is hungry.  We noticed the last break down was months ago on vacation, when she became extremely moody all of a sudden, we gave her something to eat, she was a new girl.  I know all kids have tantrums at some point, but this is like dr. jekyl, mr. hyde and is fixed by food.  She doesn't starve, she doesn't eat all junk and she asks for food when she is hungry.  So what could be going on?  Could it be a form of hypoglycemia? Could it be growth hormones? Her doctor didn't seem too concerned, but did a blood test and everything was normal.  Any sugesstions