Why could your heart flutter when you yawn?

See Below: Because in young people flutter or atrial fibrillation could be activated by the structure in the right atrium and at the superior aspect of the inferior vena cava and yawning is usually a symptom of vagal tone and may lead to arrythmias through the same process. Cold drinks with ice can lead to acute atrial fibrillation through the same process.

Random: Flutter in your chest can be related to change in rhythm and you should wear a monitor called a holter to see if this indeed is a rhythm issues. The coincidene to when you yawn is harder to explain.