When Lying On My Back And Flexing My Stomach I

When lying on my back and flexing my stomach I notice and feel a bubble like shape that the length extends from my sternum to belly button and has a 2" width. it is easy to depress and feels as if there is no mass underneath. Like an air pocket. I sometimes have stomach pain. I am 60 and go to the gym practically everyday; cardio and weight lifting routine. It does not protrude unless I flex