For your own safety and health, you may sometimes require a medical certificate issued from your doctor in order to fly. This medical certificate states that you are healthy enough to travel in a pressurised cabin and/or that you are travelling with medical luggage.
Note: Do you have an ear infection or fever? Also ask your doctor for a statement in this case. After all, travelling in a pressurised cabin can aggravate some symptoms. If in doubt, always consult your doctor.