Why is my 6-year-old female Rottweiler's belly big and breathing fast?

Poor Nicole! There are a couple of things this could be, all of them very serious. I see that she isn't spayed. She could have a pyometra, which is a life threatening infection of the uterus. The uterus becomes so swollen with infection that the belly gets very large, and it causes rapid breathing from pain. If her uterus gets too large, it will rupture. A pyometra is fatal without treatments. She could also have heart disease causing a fluid build up in her belly and the rapid breathing, she could have a tumor in her abdomen that has rupture, or she could have eaten something toxic causing fluid build up in her belly. Finally, she could have bloat. Because I can't see her in person to rule out any of these conditions and the fact that they are very serious, it's best that she sees a vet now. The vet can diagnose what is causing them and how best to treat it. Please take her into an ER vet clinic; if there isn't one in your area, there will be a vet on call for emergencies. The information to reach that vet is found on a clinic's answering machine or website.