Heart beats quickly and very hard sometimes, what is it?

It may be because of tension. For 17 ye old there is chances of heart disease. At the time of exam or any other situation like that increasing tension and rapid heart beat is common. There is no need to worry about it. Concentrate in your studies, and reduce your tension. After this again the problem exists, you can consult a cardiologist.

If this comes on suddenly like it is beating normally then suddenly beating fast, it could be a non serious condition known as svt. it usually happens in young people, i get it when i have done something to exert myself like bend over too quickly or walk up the stairs, does it stop going fast really suddenly? if it does it could be that, it could also be that it speeds up if you have been sat down too long and stand up quickly, if youre underweight it could be that you are not eating enough. either way if it keeps happening i would suggest you see your gp about getting a holtermonitor fitted, which is a heart monitor you wear for roughly 3 days to see if you have any heart arrhythmias. hope this helps, most likely given your age its nothing to worrry about but you should get it looked at either way if its bothering you.

When you stand up, your blood pressure goes up. Your heart has to work harder when you're standing up and walking around so it makes your blood pressure go up to compensate. You probably wouldn't notice if you sat up first for a few seconds, then stood for a few seconds, and then started walking. That gives your heart a chance to bring up your blood pressure gradually instead of all at once. And yes it's normal, as long as you don't start standing up and falling over. Haha good luck :)

Stress related most likely.

It may be anxiety.