The fear with a dog this age and with the symptoms that you are seeing is that the dog has a heart issue. If this is the case, you will see swelling around the chest and abdomen. This is because fluid accumulates around the heart and lungs when there is a heart condition.
I suggest that you see the vet with the dog as soon as you are able to rule this out and to get a general exam.
Heart issues are often treatable if you can catch them early enough.
Let me know if you still have questions.
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