Issue: I have ongoing discomfort in my heart.

I recently joined a gym at the end of January 2015 and have been going often (about 3 times a week). I feel fitter than I have been in over a year.

Thursday, 5th of March, I decided to go on the treadmill. I kept an eye on the heart rate monitor as I ran. I usually feel my heart get tired at 150 beats per minute, but on that day I felt fine, so I kept running until I reached 165.

I started to feel some discomfort in my chest (and possibly my left arm?), so I lowered the speed of the treadmill and eventually stopped. It's been almost 2 days and I still have discomfort in my heart.

To be more descriptive:
- My heart feels like a dull ache
- No pain
- No difficulty breathing
- No Nausea
- No dizziness

It's difficult to sleep since the discomfort keeps waking me up.

I'm not worried, but I'm curious as to why this has not gone away yet.
What do you suggest?

Thanks in advance.