It's probably down to anxiety.. I get this too more on a night time when I'm trying to sleep and once I feel it in my chest it is hard for it to go away.. I suffer from anxiety so maybe this is another symptom.. I wouldn't worry to much if its beating its doing what its meant to do.. But I know that's its scary and you think the worse but don't.. If you get so concerned and what some peace of mind ring up your doctor and ask him/her for some advice.. But ifyou don't want to you can check loads of anxiety symptoms on here and see if you fall under any of them hope I helped..
Well if you are touching your feeling your heart with your hands it's natural but if you are not and you feel it beating theres prob something wrong with you not saying you are going to die but you should see a doctor I think I hope I helped byy x