Since you have been investigated thoroughly, there should not be any concern. Please read this resource;
Though yawning is associated with boredom and fatigue, a sudden onset of too frequent yawning should be investigated for the various causes as;
1) cardiac tamponade
2) air hunger due to any reason
4) raised intracranial tensions
5) progressive supranuclear palsy,
6) multiple sclerosis
You can ask your doctor to get an echocardiography and MRI of the brain to rule out above cause too. If these are normal, medication may be the likely cause.
Please feel free for your follow up questions.