As we discussed , there are many possible causes for the signs which you describe and I am not able to rule out that these signs could be due to heart disease. 

Signs of heart disease are not uncommon in dogs and can include: 

 - Sudden or gradual reduction in exercise ability

- The dog might stop and rest when out on a walk

- Cough at rest and during exercise 

- Panting when this is not expected - for example at rest

- Fainting or falling over

- Blue (and not pink) mucus membranes 

It is important to recognise that some of these signs also can be seen in dogs that suffer from other disease such as for example: infections, pain (arthritis), epilepsy, allergic disease or intoxications. Heart disease can therefore not be diagnosed based on the signs alone.

It is very important that pets have a thorough physical examination when signs of heart disease are seen as conditions affecting the heart can cause very serious symptoms and potentially be life threatening. The investigation of heart disease may include blood tests, x-rays and ultrasound scans in order to confirm that the signs are due to heart disease and not other causes. In many cases, heart disease in dogs can be treated successfully with medication and life style changes. 

I recommend that you keep your dog rested until it is possible to have an examination by a veterinary surgeon. 

I hope that the reason for these signs are found soon.