I am sorry to hear you are worried about your Golden Retriever. My name is Rebecca. I am a veterinarian and will do my best to help.
If you check her gums, do they look pink or pale to you?
Do you know if the abdominal tumor is on her spleen? Has she had a chest Xray recently?
One concern is that one of these tumors, the abdominal tumor or the mammary cancer, has spread to her lungs. A chest xray would tell you that.
Other possible causes of rapid breathing are fever, anemia (low red blood cells), fluid in her lungs, heart disease, heartworm, and pain. I would be concerned, and if her gums appear pale rather than bright pink, I would be concerned enough to get a chest Xray as soon as possible.
Let me know what other questions I can answer, and if this is helpful.