What Causes A Protrusion Of The Muscle?

Here is one for you,as I do you situp,say getting out of bed,a muscle protrudes at the front, between the 'six pack',seems to go down,not across.

My doctor says, thats fine it happens in older people,its the gut comimg through,nothing to worry about.

Well I have never heard of it and I have never seen it in body builders who were in their 80's plus. Any thoughts please ?