What does it mean when someone says this to you?

ok so i friended this by on facebook. i am 13 and i think he was 12 or 13 idk anyway he commented on a picture when i was laying on a raft on my stomach in a pool and said "good girl" i didnt know how to respong so i put a "?". by the way i am not fat by any means im lower average. what did he mean by good girl.