3. Exercise
The main reason why your heart beats faster or more strongly is that your body needs more blood; specifically, your muscles, and more rarely, your brain. When you do moderate or intense exercise, it is normal that your heart rate increases, and even surpass a hundred beats per minute mark.
In some cases, you can feel palpitations while performing your exercises, or right after you finished them. These are normal palpitations and are no sign of alarm unless they are too strong for the level of effort performed or last for too long after your exercises are over.
If you have any doubts about your heartbeats when performing the exercise, ask your trainer or physical education teacher, or consult a doctor if you do your exercises on your own. You might be training too hard or lack the appropriate cardiovascular resistance for your exercise program, or there might even be an underlying cause affecting your heart.