it is very unlikely it spawned, how long have you had it? Try not feeding it for a shortened period of time to see if the belly does in fact go down. How big is the fish?
more than likely its belly is large from feeding or it could have a bloating issue
My figure 8 is the greediest fish! Whenever its time for feeding he's straight in there and eats till his belly is very fat. It's always flat again by morning. Is your puffer's belly staying fat constantly even hours after feeding?