It sounds as though you are experiencing garden-variety Premature Ventricular Contractions or PVCs. In general these are benign, and though it's hard to believe, *everybody* has them. The trick is that not everyone is sensitive enough to feel them.
Here's an explanation:
I know you are freaked out by this, but you are a young woman (though no longer a 'girl') and so at very little risk for anything heart-related. In addition, your ticker has been medically checked out several times. This is no joke: In general, medical tests actually do catch the biggies, and all things considered, the chances are that your heart is OK.
Consider this--and it's important: Real heart problems impair your life in a big way. If you have them, certain symptoms will show up when you do normal daily activities. So, if you can walk a block, go upstairs, carry groceries, and do it all *without* nausea, actual fainting, crushing pain in the center of your chest (as if an elephant were parked there), and a strange, drenching, absolutely cold sweat, the kind of thing that soaks your clothes instantly, your heart is almost certainly OK.
When we first experience benign PVCs, the big problem is the fright that the sensation causes, and a good shrink or counselor can help with that. Ask me how I know.
If it helps any, your heart is pretty resilient and can take a beating so to speak.
For a few years I had 54,000 extra beats and skips and my heart even stops then I'm out taking a dirt nap (fainting, syncope etc). I've had everything from Pvc's, PSVT to Vt and VTach/VFib since I was 9.
I know it's scary but if you've been checked out, it's probably just your heart readjusting to the needs of pumping and environmental things.
Keep a journal of symptoms and get a copy of all of your dr's visits and test results. This way you will be able to see exactly what is going on and what the tests say. I know for me, it really helped me when I talked to dr's when I showed them my journal.
Be informed and try to find a way to cope with the symptoms when they happen. Breathing and exercise helps a lot if you've been cleared by your dr.
Years after an ablation, I still have a ton of pvc's and other things going on even after but they found I have a genetic heart problem and had to have a defibrillator implanted.