Hi Text!
From what you put in your post it seems that you have done everything corectly getting all the medical check-ups first and finding that nothing is wrong with your body.
You have all the sypmtoms of anxiety and panic. So in answer to your question, it is all in you head. It is as simple and as difficult as that. There are as many reasons as there are people why a person seems to suddenly become overwhelmed with anxiety and panic attacks.
You can spend many years in therapy, you can spend alot of money going to doctors for test and more test, and never get an answer as to "why do I suffer from these feelings"
The important thing to know now is that you are creating these feelings by being afraid of them.
As a medical student I am sure you know the effects of adrenalin on the body. People who experience anxiety and panic go into the "fight or flight" response, they feel like they are having a heart attack, that they can not breathe, that they are sick, that they are going to die, and alot of other thoughts and feelings.
Their reactions to these thoughts, becomming more afraid, increases the adrenalin output and makes them feel worse. The next step is they associate this "attack" with being in a certain place, usually somewhere away from home out at school, in a store, at church, wherever. They begin to believe if they stay away from this place that they wont have another attack. But this is not true, because it is being afraid of the thought of having another "attack" that is what is really wrong, not the place or the people.
There are several excellent books (self-help) available on the internet, the most helpful one for me was "Hope and Help For Your Nerves" by Claire Weekes. I also use "The Anxiety & Phobia Workbook" by Edmund J. Bourne, and last recommended by my therapist is "Mind Over Mood" by Dennis Greenberger and Christine Padesky.
Stress from living day to day life and cumulative stress that has no healthy outlet I believe contribute to anxiety and panic. Finding time to relax, meditate and prayer, along with breath exercises help many anxiety sufferers.
Take care and post again to let me know how you are doing. I care.