When ever i do a lat pulldown(pull a bar downwards behind your back), sometimes bench press or mybe even just holding my self up between 2 ledges and after the exercise i puff out my chest i feel pain over on that area, i only feel this pain right after doing the exercises, then it goes away. Sometimes i feel a slight pain when twisting my upper body

I began to notice this from the very first time i went to the gym which was like a year ago (give or take a few months)
since it wasnt bad i ignored it.

my physio said the muscles joining together ontop of my sternum havnt developed yet. now i have been waiting for 6 months and i hardly see a difference

what do you think the problem is?
should i get an MRI?