I am sorry to hear you are worried about Belle. I am a veterinarian and will do my best to help. My name is Rebecca.
She may be missing the dog, but the move into a new place may also be a part of the change in her. Getting another pet might help, but might make things even worse. I always say, never get a pet for your pet; only get another pet if you yourself want one. If she has never lived with another cat, she might be overjoyed to have another cat around, or she may hate it. Same with a dog; just because she like the Schnauzer does not mean she will like a different dog.
I would start with "environment enrichment", making her feel stimulated with an outlet for her natural curiousity and need to hunt. Things like perches to look out windows, cat trees (cats like to go up high), and interactive toys (balls inside clear circular tubes that they can bat at but not reach, catnip, dangling toys) can all help.
There is a website dedicated to environment enrichment for cats:
The section on "life stressors" may help with her, moving and change are big life stessors for cats.
Let me know what else I can help with or answer. Rebecca