The light could be stressing her out. It also could be overheating the tank, as lights often do. Do you have a heater? The water should be between 76-84 degrees. Your target spot is between 78-82. If I were you I'd turn off the light. Try talking to her in a sweet, soft voice. Maybe play with her by moving your finger around the sides of the tank. Many bettas will chase your finger when you do that. Try adding some decorations from your other betta's tank, just until she gets better. How big is her tank? I'd also turn off the filter if you have one.