No, you are NOT alone. I was diagnosed with Hashimoto's about a year ago & about a month ago, started to have chest pounding & heart palpitations. Went to ER, EKG was normal, but Potassium was very low, not sure if related, but anyway, I am waiting for my appt with the cardio. I assume it will all be normal, but can't be too careful. I also felt like I was dying on a couple of occasions. I feel a little better reading your concern as it gives me hope that I will live. Take care of yourself & know that others are out here feeling your pain.
You certainly need better test than what you got at the ER
Call the PCP and get in on an emergency appointment to get the process going.
Also, I forgot to mention that when I get the heart palpitations at night I feel like I am going to die. They did an EKG at the ER and it came out normal. I just keep thinking that I have some kind of heart condition. I can't stand feeling like this.