Given your age, it is unlikely that this is a heart problem. It sounds like they are following you closely, and an echocardiogram
would be the first thing our doctors would do to evaluate for any potential problem. Since you've already had this done, and have been given the "thumbs up" then I'd say you're good to go! I would mention your discomfort to your doctor at your next prenatal appointment, and of course if it gets worse, you should go be seen sooner.