Keep her well covered, preferably with a heavier blanket, to keep out the drafts. It also helps to wrap the blanket around the cage and hold it closed with a clothes pin or a paper clip
Definitely take her to the vet, just to be safe.
Sometimes shivering and poofing out are normal behaviors (like when they just wake up or to flatten out their feathers). But BLR is very right, parakeets will try to hide an illness as best and as long as they can. If she's eating/drinking normally, playing and talking, and her poo is normal, then I wouldn't be dreadfully concerned. But take her to the vet ASAP to be safe! If it's an URI (upper respiratory infection), than catching it early and following the vet's instructions will fix it. Is there any discharge from her nostrils?