I Have Been Having A Chest Pain On The Left

I have been having a chest pain on the left side for over a week. It's rather dull and waning throughout the day. It becomes sharp with certain strength exercises, and when I lie in bed, especially on my left side, or blow my nose.
It seems to have gotten worse in the last couple of days, despite my not having exercised. I suspect it may be a pulled muscle, but why is it getting worse? What else could this be? I am not a smoker.