warning signs?

I had just finished lunch at work, Thursday, when suddenly I got this pain in the side of my chest. Felt mostly like the cramps you get when running. I didn`t think much of it at first and excused myself from class, thinking I could "walk it off". Well it didn`t go away but rather it got worse. It was painful breathing in and I could barely keep standing.

I dropped everything and took a cab downtown to the emergency room.
The doctor was not sure what this could be, so she took some x-rays and took about 3 viles of blood from me.
Before I knew it I was in a hospital bed in my own room waiting for the results (well, actually I shared a room with an older Pakistani man who turned out to be very talkative).

The blood results didn`t come back until 7 pm, but they all came back negative. None of the x-rays showed anything either.
This was all good news of course, but basically I still didn`t know what this was. Maybe it was muscular? Was it an anxiety attack? I got to talk to the doctor again, before going home, but all we could do is speculate.
Today is Sunday and I have tried to just take it easy the past few days. Went swimming for a while yesterday and that felt really good. It still feels like somebody punched me in the side, but I don`t have any problems breathing.

Maybe my body is angry with me for not training actively anymore.
I don`t really know what to do, but I am taking this as some sort of warning sign and I need to change my daily routines.