When ground grappling, my chest is very...?

Weak it seems. I begun actual mma style ground game about two years ago, with the belief that I have a weak sternum. Whenever I'm chest to chest (on bottom or opponent has side control) the middle of my chest just hurts bad. I can suck it up typically, but I find if my opponents knew - they could take advantage very easily forcing there weight to my chest. It never hurts after the match or even after we switch positions. The deeper of a breath I take, the less it hurts. (though in situations ive actually felt like tapping). Im 18, 5'10 and 145lbs. Not ripped, or even muscular. As my bench press has gone up, my pain tolerance to it has gone down. Really, really frustrating as you can imagine. Also note, theres nothing else that irritates my chest, it doesnt feel tight or anything stretching. Thanks for any ideas of why / what to do to help