Some of my females get like that. And they will eat like crazy when they are pregnant.
Ditto on peas. Also had good results with a product called Tropical Greens from API. It has fiber and contains algae, garlic, carrot, spirulina, etc. Supposed to help their digestion plus immune system. The fish love it and haven't seen the constipation problems since using it. I mix it with their normal food.
And I've quit feeding mine tubiflex worms. I know other people feed their fish these with no problems, but I lost 2 guppies in a row after feeding them these. Don't know why but the ones that died were super bloated in the belly. Maybe wasn't the worms but now it's just greens and flakes for mine, so far so good.