Sounds to me that you are DEF. having symptoms of anxiety, and the sooner you accept it as anxiety it will get better. Therapy works, and also some anxiety meds help as tools for a while to get you through it. =) Your so young (im 27 i consider myself young too haha) but you will probably grow out of this.
You mentioned this started before surgery... you were probably scared and your body had anxiety, which the feeling of chest pressure, the feeling of you cant breath etc. are VERY COMMON symptoms of anxiety.
You also said "I panic and start to think I'm having a heart attack." <--- this right here is your THOUGHTS making you scared. You really have it backwards written here. What really happened is FIRST, you "THOUGHT you were having a heart attack, THEN you panicked. This is anxiety at its best.
Every thought you have has a chemical reaction in your body believe it or not.
What you are doing is doing a pretty good job at scaring yourself. Who wouldnt be scared if they thought they were having a heart attack? But, the sooner you learn its just anxiety symptoms, the better it will get.
You've gotten PLENTY of CLEAN BILLS of health from your Dr. and theyve told you you are fine. You need to learn to accept that you are a young and healthy 16 year old. =)
People with anxiety worry alot, and with the symptoms comes more worry, its a evil little cycle. =) But you can retrain your brain to not think that way.
If you are still obsessing and scared and having anxiety symptoms, go see your Dr. and talk to her/him. You will be glad you did. =)
Visit this page and you can see all the fun and crazy things anxiety can do to you. I've experienced just about every one of them. But I'm getting WAY better with my anxiety. ;) And you will too. You'll be good in NO TIME. =)