What Is The Bulge In My Belly Button After A Strenuous Workout?

After doing a high volume sit up workout yesterday I notice a small bulge in the shape of my belly button. It is painless, just a little squishy to the touch. IT does not protrude out of the belly button, so I still have an "inny" but the wall of the interior has bulged out a bit. I only noticed it by coincidence. Is this something to be concerned about?