Can you Help me and my border collie puppy?

ok i got my border collie ouppy at 10 weeks and the people said she is a purebred but since i got her she hasent grown an inch i mean i have been ginving her exersize and lots of food and sleep and everything so is there somthing rong with her or maybe is she a mixed breed?? She is very tiny and only ways about 5-9 pounds!!!