Welcome to Yorkie Talk!
I would definitely take her to the vet to find out for sure is she is pregnant and for xrays to see if there are pups, how many, and if they will be small enough to fit through her birth canal.
If you suspect that your female is pregnant....or even when you had planned on her getting pregnant, you should have all the whelping supplies (heating pad you mentioned, & other things) on hand long before she is due. FYI for this pregnancy (if she is) and/or for later, check out this wonderful & informative website
Canine Reproduction Video and Book on Breeding and Whelping dogs by Debbie Jensen for dog breeders and Whelping Supplies needed to deliver puppies.
Good luck to you and your girl; please keep us posted
I love belly pictures, pregnant or not.....so post away! Or would love to see even just a picture of the mama to be.