Thanks guys, for all your contributions.
I have done my own research and come up with my own possible causes.
I have two probable causes.
1. Costochondritis: Inflamation/tear of the cartilage at the sternum.
http://www.mayoclinic.com/health/costochondritis/DS00626I suspected this, but my doctor says I should feel the pain at anytime of the day, not only during running. Anyway, for this I need to stop running for sometime so that my heavy breathing do not aggravate it. This is what I will do now, stop running/brisk walking for about a month, and see what happens.
2. Hiatal Hernia: Stomach pushed up through the daiphram.
http://www.mayoclinic.com/health/hiatal-hernia/DS00099 For this, looks like my running days are over. Surgery is needed to correct this, and I do not want surgery. No matter how minor it is. No to cutting my body unless it is absolutely necessary.