I think you should take her to the vet.
She could be pregnant, or she could be having a false pregnancy.
If she is pregnant she should be seeing a vet, and if she is not pregnant you need to take her to the vet to see why she is bloated.
There are all kinds of possibilities why a dog could have a bloated belly. Could even be worms. But you also need the vet to rule out "bloat" which is a deadly condition. I hope you just meant distended and not bloated.
Here is a website with some conditions that could cause a distended belly.
Dog Medical Conditions: Distended Abdomen in Dogs
And when you get new dogs you should always get them vet checked.