my bird stays on the bottom of the cage at night...any idea why?

If hes a new bird then hes probably nervous. I had the same problem with mine (coupled with diarrhea) a couple of months ago and because I had no clue I took him to the vet and he informmed me that it was due to stress.
After a few days he was fine and dandy, just keep an eye on him to be on the safe side.

Do you think I should take him to the vet or will it be okay if I wait it out and see how she acts in a couple days? Because I know that parakeets try to hide how they feel, and I dont want anything to happen to her.

Try covering part of the cage with a towel. Leave a corner open so you can see into the cage when your in the room. The idea is to give it a chance to become used to the cage without feeling so vulnerable.

New cages, especially in a new enviorment take some getting used to. Giving them a sheltered corner to hang in while they get used to all the new sights & sounds seems to speed up the process.

Also make sure to arrange perches so they can jump from one to the next and get to the top. Most keets quickly learn that if all else fails they can climb the cage walls itself.