My grandson is 16, eats like several horses, and is desperate to put on weight. He is so thin, you can count his ribs, if he sucks his tummy in, you can probably count all his veins bless him. But he's a home body, eats normal hralthy meals, added onto that tons of junk food, but he tells me he wears about five layers of tops and hoodys to bulk out a bit, and he still looks like he would fall through a crack in the pavement. He went through a chunky stage at 9-10, but since getting this skinny, he has been seen twice by the Doctor (only to appease his Mum!) and he said its just how he's made!
Then I remembered my cousin being the same, rake-thin, yet he hit mid thirties and its like every calorie he had ever met jumped on board lol. (He was happy though lol)
I do hope you find out that all is normal and your GD is ok. xxx