Strange muscle pain

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I have recently started doing some core strength training and at class the other day I had the most weird 'event' I was doing bench presses when suddenly I felt like somebody had hit me in the chest with a cricket ball - it felt like I had been whacked right in the sternum and it made me shout in shock, much to the consternation of others in the class!

It wasn't muscle pain like pulling or tearing a muscle. more like a thump and I am not even sure whether I heard a 'thump' It is a bit tight now so I am going to lay off any exercise around that area this week.

Anybody any idea what it might have been, I have searched the internet but not come up with anything?