Something else is going on with Maisie. This sounds like a syncopal ("fainting") episode often associated with arrhythmia (irregular heart rate) of advanced heart disease. Boxers are at risk for cardiomyopathy (heart muscle disease). Please let Maisie's vet know what you witnessed. I suspect that X-rays of Maisie's heart and lungs, an ECG, and perhaps echocardiogram (ultrasound of her heart) will be recommended. Please respond with further questions or concerns if you wish.
Not necessarily. Arrhythmias come and go. I might have to have my patient wear a 24-hour halter to be able to pick up the arrhythmia. Please continue our conversation if you wish.
Activity might incite an arrhythmia - anything that increases the heart rate ("stresses" the heart). Treatment would be predicated upon the findings in the tests I mentioned. Bradyarrhythmias (slow irregular heart rates) often require pacemakers. Tachyarrhythmias (fast irregular heart rates) can be addressed medically.