Do you think my dog is premature??

i have a female rottweiler mix with german shepherd. i think the father was the german shepherd and she is full grown 2 years and a half now but she is very light she weights 50 pounds and i dont think that is normal also when i bought her the lady at the store recomended me toothpaste vitamins somethng like that and i did gave her a full tube two full fingers of vitamin about for a month here is the link to the you tube video of her please tell me what u think oh i also tought maybe she is german shepherd mix with another dog with rottweiler colors but smaller with there are some she is colored like a rotweiler and shaped like a german shepherd.. thank you http://youtube.com/watch?v=Qa8Iuv5YGvI