first of all, none of us are doctors and you are underaged. contact a physician.
but, it sounds like you're having panic attacks. this is what i went through three years ago, and still do to this day. but please keep in mind that everyone is different.
Don't rely on ER doctors to diagnose problems, unless you have a real emergency, injury, heart attack, bleeding wound, etc. You need to see an Internist, and if you don't have one, ask friends or family for a recommendation, or call your nearest clinic, explain what's been happening, and to see someone ASAP.. They will generally get you in to see someone as soon as they can. Another option is to go to a clinic that an Urgent Care service onsite. They may make a referral to the MD they feel is appropriate for you, and that will get you in sooner, too.
I had almost the exact same things happening to me. Finally my PCP sent me to a blood specialist and he found out my that my B12 was depleted. And my body could not absorb it thru food or the vitamins (pills or the liquid that you put under your tongue). He said he did not understand how I had kept from passing out. I just felt dizzy and a overall bad feeling and so so tired ! He had the nurse show how and where to give my self the shots and within a week I felt better and the second week I felt like my old self again. Sometimes this just starts happening to some people. Usually older adults. I was only 40 but I had several major surgeries and they think that might have brought it on. I would have them check your b12 level for sure !
It isn't insane for your age to have heart trouble. Sometimes you can have an arrhythmia. It may be benign (needs no medicine or other medical attention) but you may have one or a structural problem. firstly, to double check you had your fasting blood sugar, and blood pressure done and they were normal? Also, it could be a panic attack that comes on and your heart races and you feel like you are going to die. That is a symptom of a panic attack. So even if you are not stressed you can get this. There are ways to treat panic attacks such as biofeedback and medications. So, keep that in mind because it is possible especially at your age and I have heard a lot of 20 year olds with this too. So, the next thing is also to rule out that you don't have an arrhythmia. A serious one usually shows up on an EKG if they did one on you. Did they do an EKG? I had an arrhythmia and I felt my heart pounding and felt like there was flutter and I would cough or sometimes it felt like it would skip beats and I would get of breath and it would speed up. I went to my doctor and they had me go to a cardiologist to have the nurse put on a 24 hour monitor that you wear even at bedtime. It has wires that they stick to your chest and it doesn't hurt. You bring it back after 24 hours and they take a few days to read the results or maybe a week. It reads the electrical part of your heart that maybe the EKG doesn't pick up in the short time that they run that test. The most common are benign arrhythmia's and they don't have to treat you with medicines these are called PAC's and PVC's for short. Then there are other types of arrhythmia that have to be treated with beta blockers or other drugs such as atrial fibrillation and Tachycardia. The other thing you can do is to get a echocardiogram. This is to check for structural problems. It is a sonogram of the heart and checks the structure of the valves and so forth for things like mitral valve prolapse. I really think this is probably a panic attack that needs to get under control. If it is I don't know why you are getting them. But it is best to get diagnosed with the right thing or you could miss something important. I hope this helps.