If you have a QT, move her into it. Epsom salt (can buy at most stores with a pharmacy), use 1tsp per gallon, 100% water changes a day for roughly 10 days or until she goes down. Fast her for at least 2 days, after that feed her 1-2 pellets a day until she stops bloating. Bettas will over eat, and being in a community tank, she will eat more then what she should due to you not being able to control it. They will kill themselves by over eating. In the wild they eat insects, and they fast for a day or two every so often on their own.. why we should do a day a week for them to help them.
What other fish is she with? And what type of food are you feeding?