Depends how many episodes you have in a day. if it is frequent than it's a matter of concern.
Avoid risk factors, smoking
do physical, eat a fat and decrease salt diet etc.
As the causes are too many and you will have to consult your doctor.
means to become aware of one heart beating, normally at your age it must be over 100 beats /minute (but don't take it as a parameter). EKG is the best way to diagnose palpitation, echocardiogram
During menstrual cycle it is common in females.
But I think it's normal at your age.