The yellow material may be yellow urates, normally the white in the droppings. Her lower belly or the bulge could be from swelling of internal laying or water belly. Does there seem to be any crusty material inside the vent, or do you feel any stuck solid droppings? I would offer some chilled coconut oil cut into tiny pieces, and offer her some to peck up to 2 tsp per day, in case of constipation. She may have either internal laying or another reproductive problem.