So sorry!
Have you tested the water? I would usually do a good water change or two at the first sign of trouble, just in case. Something may have gotten in the water--a spray or something. I just had one of my new sorority girls die. I have no idea why. I did a major waterchange, added some salt and today some IAL. The others are doing fine now, and the tank looks good.
Betta do well with a small amount of salt, and Indian Almond Leaf is a highly recommended additive for Betta and other black water fish.
All you can do now is keep it clean and watch. Also fast the girls a day or so and then give them a bit of smushed blanched pea or daphnia.