What could be wrong with my little cousin?

Shes 6 months old and is still the size of a 2 week old. Shes alert, but stays rigid and doesnt move her arms or legs, just keeps her hands balled up at her chest. The doctors cant figure out whats wrong with her. Shes fed well, so they arent starving her, she just doesnt grow and still wants to eat the same amount as when she was 2 weeks old. Her arms and legs are so tiny, and her body just stays stiff all teh time. She doesnt move her head or laugh or smile, just looks around, cries when shes hungry. Shes not retarded and she doesnt have downs or anything like that, but we're so worried, does anyone have ANY idea what these could be signs of?