Hello and thank you for asking your question. My name isXXXXX will try to help.
Lizzie is having a "false pregnancy."
When a dog ovulates the egg, the structure that produces the hormones to maintain a pregnancy can remain high despite the female not being bred. So the high hormone levels make the dog feel pregnant. They can have mammary development, increased appetite, nesting behavior, and restlessness.
It usually resolves in about two weeks.
If she is really anxious, your vet could prescribe a tranquilizer for her.
It can happen again, after she goes in heat.
Spaying can greatly reduce the chance of mammary cancer. It will prevent uterine infections. It will also prevent the false pregnancies from happening again.
I hope this helps.
If you have additional questions, please click "reply."
Thank you. That is what I was suspecting. I appreciate the quick answer. It really does help to know for sure.