Hi and welcome to the forum! I am glad that I can help you.
You want to know the cause of the symptoms that you experience.
When people exhale the diaphragm starts to contract in order to help the inhalation. When individuals exhale and hold their breath, the diaphragm quivers ready to contract and move downwards in order to draw air in the lungs. Also, when individuals hold their breath, the heart rate lowers a bit.
According to the data that you described, it is not likely that you experience any medical condition. If you experience other symptoms such as shortness of breath, coughing, chest pain or general lividity of the skin, see a pulmonologist.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact your doctor for further advice and information about diagnosis and treatment options.