what is it?

any mummies out here experienced some kind of 'flat tummy' out of the sudden? i was lying on my side ystd and baby was very active moving inside. my partner switched on the light and then i realised my belly was gone(i haave prominent preg belly) . i then took off my dress to see and saw my belly went from normal to 'flat tummy' like the pic i drawn over. i was scared and told to sit immediately and it when back to normal after few seconds. it is normalllll???? am at 29weeks