Abdomen is belly, it can't be seen really but felt. Sort of between her legs.
Bird pellets.. layer feed? At her age and not laying often.. too much calcium is not idea.. I might consider switching to a "grower" feed since excess calcium can contribute to gout (and kidney failure).
Do you know what her breed is? 4 years is a ripe age for some.
She is very likely a bit lonely.. which definitely could contribute to reduced intake or even depression. Did she out live all her mates?
Is it possible you just didn't see her pass any droppings? Check in the morning where she's sleeping now to see if there are fresh ones.. I've never yet seen a bird not broody or bound go overnight ..