Thanks for the info. It must have been pretty scary to see something like that happen to your dog! What you saw definitely could have been a seizure, but it can be very difficult to tell the difference between a seizure in a dog, and syncope (which is essentially fainting). Causes of syncope are usually related to the heart not pumping enough blood to the brain. The result is not enough oxygen gets to the brain, and the dog will faint. Sometimes this can also lead to a seizure.
Your dog's heart rate is very fast (166 beats per minute). For a dog the size of a boxer, the normal resting heart rate is around 80-100. If your dog was just exercising, I would not be too worried about this number, but if she continues to have a rate this fast while resting, I would be concerned. Boxers have their own type of heart disease that is called "Boxer Cardiomyopathy". The disease is characterized by abnormal electrical activity in the heart, and often you will get a fast and irregular heart beat. Given that your dog's episode this morning happened after exercise, and that she was coughing just before makes me concerned about a possible heart problem.
The way to diagnose this disease is to have an ECG tracing done. Sometimes the irregular heart beat can come and go (which is why the vet may have not picked it up this morning), and sometimes your dog needs to wear a holter monitor (which is a monitor that records and ECG for 24 hours). An ultrasound of the heart (echocardiogram) also may need to be done. Usually this needs to be done by a veterinary cardiologist.
There are other possibilities for what is going on with your dog, but a heart problem is something I would definitely want to rule out. Unfortunately, this means that I do have to recommend that you have your dog checked again by a veterinarian. It is difficult to say whether or not you have to rush back in to the emergency clinic though. If your dog seems stable (i.e. gums are pink, dog is bright and alert, eating and drinking), then it is probably something you can make an appointment for tomorrow with your regular veterinarian. If however, your dog appears lethargic, is coughing, or has pale gums, cool extremities, or is not interested in food or water (when she normally should be), I would recommend that you go back to the emergency clinic.
In the meantime, I would try to keep your dog rested - don't take her for long walks or get her worked up until you have seen a vet again. Also, if you have a video camera, you may want to keep it handy, and if your dog has another episode, try to record it and bring it with you to the vet. This can be very helpful in making a diagnosis.
I hope this information helps, and I hope everything works out with your dog. Please let me know if you have further questions.
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The information in this post is not meant as a diagnosis, and can not take the place of an examination by a veterinarian.