this happens all the time to me, overnight, but it goes back to "normal" soon; for me, it's other things in my abdomen, like my bladder, intestines, water retention.. (even before pregnancy i used to get big fluctuations in my belly size overnight too).

my belly grows on the whole very gradually.. so how exciting for you to get a "pop"! that's what i want to happen to me, but mine never lasts..