This Morning's Scare

I sleep late at night 1am. Cos, thats just how it is. I wake up at leave at 7am and then I work-out

Just like everyday I started working out at 7am. I have been having a slight discomfort in my back so I decided to do just two intervals instead of three.

Once floor was done. I hopped on to the treadmill only to find out that I felt very suffocated. I stopped in just one minute--waited an started again. I could barely do ten minutes.
I felt my heart was about to burst out, I could not breathe.

I went to see my doctor, and guess what? All is great.
I still don't know why I felt breathless or suffocated. Maybe I was just too tired???

What do you think?