It could be either a true pregnancy or a false one. If you want to know for sure a trip to the vet can confirm. The vet can tell as soon as three weeks into the pregnancy by doing an x-ray. Which may be the right thing to do in order for you to be totally be prepared and see to it that she gets the proper care.
Here is a helpful site that could help you understand a little better on what signs to look for.
4 Ways to Tell if a Dog Is Pregnant - wikiHow