what makes a dog seem as if his heart is beating very fast after walking a normal walk?

Question

he is three and a half years old he is a silky terirer
he lies awake on the floor and his heart is just beating fast and he seems to be distressed but lying quietly

Answer 1

Dog's heartbeats are faster that people's, so "very fast" is relative.... for a dog that size, even a 100 bpm can be normal.

if he is distressed easily, then you should take him to the vet. It can be heartworms, or he may have a heart murmur, heart failure, or he can simply just be out of shape. Better to make sure he is ok.