Sweetpea is such a naughty little girl for hiding on you.
I swear she must be related to Samantha because she disappeared on me a couple of days ago too and I looked in all of her favorite places and finally found her curled up sleeping on a shelf in the basement that she hadn't been on in 2 years!

It could be that Sweetpea is looking for a cozy safe place to have her kittens like others have said above, or it can also be partly due to the cooler weather.
Do you have an estimated due date for her kittens?
I'm sorry that Sweetpea gave you such a scare.
Give her some hugs and kisses from me, she's such a little cutie pie.