My fish Taco seems to be constipated for the second time. I feed him 2 pellets in the morning and a freeze dried bloodworm or two in the afternoon. I recently stopped feeding him the freeze dried bloomworms as soon as I saw him getting the belly. I'm not sure if it's happened to anyone else but I think they're causing the constipation.
What I do is the pea thing. Put it in a microwavable cup with water and microwave for 2mins. I take it out of the shell, put it on the plate and mash it/cut it up with a toothpick until they're about the size of a pellet. I feed him a little less then half the pea. They seem to sink really fast so he doesn't get all of it all the time. You can try not feeding him for a couple of days to see if it clears out on its own. I read that epsom salt can help, like 1fish2fish said. I also read that you shouldn't use aquarium salts when betta's are constipated. It makes them absorb water. Hope that can help.