My cat is almost 20 years old but in wonderful health. I walked in on him and he seemed…

This could be multiple things and ideally your cat should be examined by your veterinarian. You don't describe the attack but I would be suspicious of a seizure. Seizures can happen for many reasons. Cat's can also have asthma like attacks. Even at 20, many conditions are treatable. For example, high blood pressure can cause strange symptoms like you've seen and it can be treated easily. Hopefully you won't see one these attacks again but to be safe I would make your oldster is o.k.!