What is wrong with my cat?

She is 7 months old, and very skinny and small, but she was always that way. My friends joke that she hasn't grown any and they say she is a bag of bones. Well i recently moved into another apartment and she was fine the first month of so but she has slowly turned into a complete terror. She meows all the time, she begs for food so much that i can pull her away from my food and she just goes right back, and she has gotten even smaller. she seems to be eating her dry purina kitten chow... i have no idea what to do! She has turned into an insane back of bones devil!