Sounds like a feeding issue. Is she swollen behind her pectoral fins at all or anywhere? I would feed her a pea treat and see how she does with nothing but peas for 24 to 48 hours. Sounds like she may be starting to be constipated. When my bettas got constipated the first sign was a loss of color. Don't give her betta food for a couple of days just peas and make sure the tank is at 80 degrees to 82 degrees to help her get her digestion moving.

They are not always swollen if the problem is not severe or long lasting so don't go just by that. Just try the peas for the couple of days and see if she perks up in color. Do please keep us informed as I think this will do it but if you notice any other symptoms we want to know.

Rose