Canine Reproduction Video and Book on Breeding and Whelping dogs by Debbie Jensen for dog breeders and Whelping Supplies needed to deliver puppies.
This is my favorite website for information on everything from heat cycles, mating, pregnancy info, and whelping, all the way through weaning the pups. If your girl is only on day 31 since mating, she is still VERY early in her pregnancy and it will still be hard to tell, though signs are enlarged teats & her vulva will still be swollen. Morning sickness is a sign (mine had it for a week or so the first week after mating), but not all females experiance this. A vet or experianced breeder would be able at this point to palpitate her belly and possibly feel pups, but this should be done by someone who knows how to do it. X-rays are not done until about a week before she is due to whelp as the puppies bones are not calcified enough this early to show up on the xray. If you even suspect that she is PG, yes, you should be feeding her a high quality PUPPY food.....as much as she wants to eat.
Good luck and please keep us posted on whether she is pregnant.....or not.