Eira, 7 weeks is far too young for a chihuahua puppy to leave it's mother - it mightn't be fully weaned and really needs the socialisation it will get with it's litter mates.
12 weeks or at the earliest 10 weeks is the recommended age for chihuahua puppies to leave their mothers and litter mates - see can you talk the breeder into keeping her a few extra weeks - it really will pay off!
If you already have her, be very sure to feed her frequently as Evelyn says, and be sure you know what to watch out for with hypoglycemia - low blood sugars, have nutrical or somesuch handy - search nutrical here on the forum and Hypoglycemia for hints on what to watch out for and do if the pup does seem shaky or lethargic.
Here's an article on hypoglycemia -
Article on hypoglycemia