Your only 19, way too young to have any kind of heart problems. The fact that your heart tests were fine should give you the peace of mind that its only anxiety. There is no way the tests wouldn't pick up any issue with your heart and nothing could of happened in those two months that would suddenly cause a healthy heart to not function properly. The next time it happens just know that its anxiety. Do some deep breathing, focus on something else, and if you ignore it it WILL go away. I'd suggest trying that first for a while and if it doesn't help you then talk to your doctor about increasing your medication dosage.