Caring for a 6 Week Old Puppy?

I have a 6 week old puppy; she is a Chorkie. I got her early as her mother was not well enough too look after her. So I am feeding her powdered milk and puppy meat. She's been eating and drinking fine, but the past 2 days she won't eat anything. She is drinking the odd bit of milk, that's it. She is OK herself and is playing and running about, but looks a bit dopier than normal. Do I need too worry about this? X