I've only heard of bettas pooing white for two reasons, one, parasites, or two, females when they get eggy, they can either absorb eggs, or release and eat them.
Generally betta poo is whatever colour their food is, and often looks like a cinnamon bun (lovely analogy I know).
IF you could fill out this http://www.bettafish.com/showthread.php?t=49233
it will provide some better back ground.
As well this is a bit of info on internal parasites courtesy of Darkmoon.
•Symptoms: Betta is losing weight but eating normally and acting lethargic.
He/she might dart or rub against decor.
•Treatment: These can be hard to fight and can get confused with the fatal disease Tuberculosis. Perform daily 100% water changes (if possible, for larger aquariums change 3/4). Make sure you carefully clean the gravel to remove eggs/larva. Aq.Salt does not seem to be affective against internal parasites. I find combining ES with an anti-parasite med is best. Treat with 1-2tsp/gal Epsom Salt combined with either Jungle’s Anti-Parasite Pellets, Jungle’s Parasite Clear Fizz tabs or API General Cure. PP is also effective against internal parasites.
Since a fairly conservative treatment for bloating/constipation is epsom salt, I would advise you to try that treatment. Epsom salt is supposed to help with bloating, and help them poo.