Yes, she needs to be seen by a vet to confirm pregnancy. A vet can tell after a month by palpation. X-rays can be done at around 54 days. Until the time that you know for sure, treat her as if she is. Make sure she is eating well, no heartworm or flea preventatives, limit jumping. Here's a site that will acquaint you to what you can expect if she's pregnant.
Dog Breeding and Canine Reproduction by Debbie Jensen. Dog Breeding and Whelping guide for dog breeders.