If the area at her "belly" is getting fat, then it's eggs or constipation. Her belly is essentially where her ventrals are, where her ovipositor is, right behind her head and underneath her chin, and in front of her anal fin. If it's constipation, then getting her to eat a pea should take care of that. Peas are fiber...Betta laxatives. If it's eggs, then she'll probably release them or reabsorb them on her own. There's a small possibility that she'll become eggbound, which, unfortunately, can be a fatal condition (I lost a dear girl to that), but it is a small possibility, and most girls either drop the eggs just fine, or reabsorb them. Rarely do you see the eggs after they've been dropped, unless you arrive during or after, as the girls and their tankmates find Betta eggs to be a very nutritious snack.
If the whole of her body is enlarging, then it is probably dropsy, which is usually a fatal condition. I have no advice on dropsy. Only fish I've lost to dropsy was a pregnant Guppy near to her term and her fry, which was why I didn't diagnose and treat in time. Dropsy is characterized by a "pineconing" of the scales...if viewed from above, the scales would be puffed out at angles, giving the fish the appearance of a pinecone.