I know the symptoms are scary to observe, but not life thretening.
We can not diagnosis this defnitively in this venue. for that, you will have to take your pet to the vet.
The cause of this could be completely benign or it may be an illness or condition that requires immediate attention, in order that your pet can have the best of care.
The things that come to mind are:
Of these, the most urgent issue is the heart worms.
since I am not able to examine the pet in any way, let me show you a link to heart worms infectiosn so that you can make your own determination of the urgency to get your dog to the vet.