are you kidding?? ok.. not trying to be rude.. but if you know they already tied, why wait until right before she delivers to wonder if she is pregnant? Just trying to understand. Anyway, absolutely she more than likely is due to have puppies SOON. Diapers and belly bands will NOT keep dogs from mating. Has she been to the vet yet? How big is she and how big is he? what are thier ages? I am going to assume for the time being that you did NOT INTEND them to mate. If that is true, is there a reason why one or both were not spayed/nuetered? Looks like you need to learn a whole bunch in a short period of time. check out this website
Dog Breeding and Canine Reproduction by Debbie Jensen. Dog Breeding and Whelping guide for dog breeders.
study the links on the far left of the page. It will help you gather up your supplies and help prepare you. Good luck. Hope she has a safe whelp
ps.. I just noticed you are NOT a NEW member but have been here a long time.. That makes this even more puzzling to me. I really do wish you the best with whelping