Since you've only had your kitty for a few days, she definitely could still be in an adjustment period. It takes some cats weeks to be comfortable enough to boldly venture out. Every so often, you can try to entice her out of hiding with some treats or toys. Cats are by nature nocturnal, so you might just need to see if you can adjust her schedule to yours. Sometimes I still hear my cat running about in the middle of the night. He likes to nap a lot during the day so I usually try to exercise (entertain) him right before I go to sleep so he's not too bored at night. Boredom equals mayhem to things not nailed down in my place!
Has your cat been checked out by a vet recently? Was she vaccinated recently (sometimes cats can be a little less active after certain shots)? As long you see evidence that your cat is using the litter box, eating and drinking, then she's probably just getting used to things. Cats have amazing bladder control when they want to and can even go a few days without eating; however, if after the three days that you've had her there are signs that she isn't active at all and isn't going about daily 'activities', you may want to take her to a vet to be sure there aren't any health issues.
I don't know about Siamese hair qualities but there are others on this board who have Siamese who can probably help you.
I hope your purrball adjusts well