I agree with JGF completely. You describe textbook panic attacks IMO. I also agree that you've developed health-related anxiety as well. It's not uncommon to have an onset of anxiety and panic after a major life change. You're halfway around the world, and while you may have been excited and not at all anxious about it initially, sometimes there is a bit of a delayed reaction. I've always been anxious, but my panic disorder emerged officially when I was away at college. I was away for months, and did fine, not anxious at all, and one day, BOOM! It hit me like a ton of bricks. The onset of an anxiety disorder is also most commonly seen in people in their late teens, early 20's, so you fit the bill pretty well.
A few things...while panic attacks are terrifying and while the symptoms are very real and very scary, in those moments, you have to try very hard to remind yourself that FEAR IS NOT FACT.
Remember that you've had a very thorough medical work-up that came back perfectly fine. Remind yourself that you're 24, and healthy. Major, life threatening heart problems are super rare in people your age to begin with, let alone in someone who is healthy. Also, if there was something big and bad going on with your heart, it's almost guaranteed that the tests would have showed SOMETHING. Don't focus on the symptoms and how you feel, or the "what ifs", but rather focus on the FACTS, what you KNOW to be true.
Anxiety and panic actually cause some benign cardiac symptoms, and I would bet that's the weird sensations you are feeling that's causing you to worry about your heart. One is palpitations, where your heart will race and pound, the other is PVCs, or premature ventricular contractions. PVCs are basically an "early" beat, and it's completely harmless. I've had them too, so I know how unnerving they are. It does feel like you're heart has either stopped or WILL stop. I would bet that that's what you have experienced, PVCs. Did the doctor mention anything about that? The medication you were put on will help with those kinds of symptoms, but I think you also need to formulate a more comprehensive anxiety treatment plan that would involve mental healthy professionals.
You need to schedule an evaluation and discuss treatment options. Therapy is very helpful to start with, as it will teach you not only ways to cope with panic when it hits, but also how to change that "what if" thinking cycle you get stuck in. There are also many different kinds of medications that can be helpful in managing the symptoms and panic attacks while you do the work in therapy. Being oversees, you would benefit from ordering a few self help books to educate yourself about anxiety, and doing some work on your own. The one book I recommend people start with, it's been a bestseller for YEARS and is very helpful is "The Anxiety and Phobia Workbook" by Edmund Bourne. There are TONS of other books out there as well. Anything by Claire Weekes is great. Here's a link to a resource page I compiled with many book suggestions. Browse through it and maybe order a few:
http://www.medhelp.org/health_pages/Mental%20Health/Anxiety-and-Panic-Resources--Books--Websites--Self-Help-Programs/show/1285?cid=60
You also have to help yourself as well. Like mentioned above, there are lots of self help books and websites geared to helping you learn methods to address anxiety. Also, you have to start avoiding the activities that you know are worsening your anxiety...do not search the internet for health information, symptoms, diseases, etc. That will fuel your anxiety terribly. Also, steer clear of anything that may trigger or worsen your panic, like stimulants (caffeine), alcohol, etc. Alcohol is AWFUL for anxiety, so if you drink at all, I would recommend cutting it out completely while you're working to get your anxiety to a better place.
I understand the urge you have to come home to be evaluated by doctors without the language barrier, but my honest opinion is that that will actually be more harmful than helpful. One, for the obvious reason, it will interrupt your academics, which you will have to then work to get back on track, and two, the simple act of "fleeing" to a more comfortable, "safe" place will reinforce negative thoughts to your mind. It will basically send the message to your brain that you're not safe and healthy in China, which can (and probably) will lead to you fearing going back, which would be a hinderance in a big way to your future goals. I think it's AMAZING that you're studying in China! What a once in a lifetime experience! What are you studying btw?
I would recommend treating this as anxiety where you're at. At this point, from what you've described, it's my opinion that you don't need any further medical work ups as it relates to your heart.
I wish you the best, please keep in touch and let us know how you're doing okay? Hang in there, and know that you are far from alone. Here, you're among people who understand and have gone through the same thing you are going through.