Hi. It is your doctor that has to find out what is going on but it could be a number of things. It can be a muscular-skeleton pain. It could be heartburn or GERDS. It could be a gallbladder that is acting up from time to time. It would be VERY unusual for it to be heart related in a 19 year old but possible. The good thing is that you've had a few tests and they showed nothing wrong with the heart. 
I would suspect a muscular-skeleton thing. Have you been working out recently or lifted some heavy things?
Now as far as feeling like your heart is skipping a beat, that could be simple premature heart beats. It isn't skipping them. It is just throwing in extra ones here and there. It happens to ALOT of people and is not anything to worrying about in a structurally normal heart. Some people do not feel these type of beats at all and some people feel EVERY ONE. In fact, it is not the extra beat that you feel but the one that follows it. This is because the one that follows is usually pumping out a bit more blood since the heart chamber has a bit more time to fill up than it did on that extra one.
These things should calm your mind since stress and worry will only make you have more of these extra beats. Also you have some things/ideas that you can ask your doctor if you continue to have problems.