Puppy Not Eating Regularly and Losing Weight?

I got a puppy almost a week ago. She went to the vet the next day and had her first shots and the vet wormed her too. She didn't seem to want to eat too much when we got her, but was weighed at 3.70kg. She will be 8 weeks old on Sunday and she doesn't seem to want to eat and has had soft stools for 2 days. She is still drinking and is very playful and she will eat occasionally, but seems to be losing weight. I can see her hip bones when she is standing. Please help. I am very worried and not sure if she could be fretting for her mum and siblings.