A month after being bred, maybe a little more for smaller dogs, an experienced person can feel puppies. There are ultra sounds and X-rays to be sure. False pregnancies are fairly common. It is entirely too possible somebody knew she was bred and dumped her. They may also have responded to a false pregnancy that way.
I agree on checking with vets and shelters. It is possible somebody is indeed looking for her and may not see your posters. Also, have one of them scan her for a chip. And yes, shelters and rescues will take pregnant dogs. Your shelter should have a scanner and perhaps somebody experienced enough to make a good judgment on her being pregnant.
You may find a rescue near you starting at
http://www.akc.org/breeds/rescue.cfm