Is he still eating/drinking ok?
Has there been any vomiting or diarrhea?
Lift his lip, what color are his gums?
Does his belly seem distended at all?
I am not usually an alarmist, but I would be concerned that your dog is either having some sort of heart issue, or there is a tumor in his abdomen that is starting to cause him problems. Any time I see a 12 year old golden with sudden weakness and rapid breathing, I am very worried.
I would strongly encourage taking him to an emergency vet as soon as possible. The earlier that he is treated, the better the prognosis.
I may be wrong, but based on all the information you have given me, I would suspect something very serious is happening.
Please get him checked out ASAP.
I hope this helps.
I don't know if you saw what I added but he did throw up (very yellow) but water looking. now he has laid back down and resumed his quick breathing.
Yes I saw that, and that only adds to my concern.
I hope that you take my advice and take him to the vet immediately.
Good luck and let me know how he does.