I think it depends on the 'shape' of your belly button!

Mine went flat last time too - with just a little bit poking out at the top LOL I used to stick a bandaid over it so you couldn't see it through my shirt. It didn't 'pop' like that until about 26 weeks I think? This time around, it has already gone the same as it was at 40 weeks last time - I guess it remembers what is happening??