Disconfort Between Centre of lower Chest Area

Hello
i started to exercise and after some days i start Abdomen exercise but last week when i finished doing it then i started to feel little discomfort at the centre side of Chest and opposite to Stomach area and i feel discomfort while i m Sitting (like some think is trapped between chest and abdomen area which only feel at the lower centre side of chest and opposite to Stomach area and i cant sitt comfortable for long time also my muscles at that side are produce pain when i touched them my Doc says that its just Muscles Pull as he cheked me but i think its something else can any one help me about it its will be gr88full