I've had ongoing chest pains and irregular heart beats that have scared me enough to finally see the doctor on two different occasions. Both times I feared that my heart might stop. Like you, I also never experienced any shortness of breath. Plenty of tests later when nothing bad was found, I was told stress and/or acid reflux may be the problem. I worked on correcting these issues as much as I could. It's been years since then, I still have occasional symptoms and my heart is still working fine, I'm 50 years old and have always been at a healthy weight and active.
By all means see another doctor, or even two if that's what it takes, but I know from experience that it is easy to sike yourself out worrying about this, and I won't be doing that again. I also know that when the doctors tell you it's not a heart related problem, they know what they're talking about. Pedal On!