Sinus arrhythmia is not considered a heart disease. As a matter of fact, it can exist with no significant damage to the heart tissue or irregularities in heart work.
Sinus arrhythmia indicates a slight acceleration and slowing down at the same time when breathing in and out. It is more common in children, but adults can be affected too.
The chest pain you experience might indicate that another process related to the heart has occurred. If the pain is severe and appears more and more often you might want to consult with a cardiologist for further examination, one of them being echocardiography.