Aw, poor little girl.
As Corsica said, Epsom salt is a safe treatment. Epsom salt is magnesium sulfate. It acts as a laxative (in people and fish), and helps reduce fluid retention. Therefore, it may help with bloating, constipation and buoyancy issues.
Epsom salt is not "aquarium salt." (Aquarium salt can only be used for short periods of time, as it contains sodium, which contributes to fluid retention, puts stress on the kidneys, and can lead to eventual kidney failure.) Epsom salt is magnesium sulfate. Since it contains no sodium, it doesn't put stress on the kidneys, and is safe to use for internal issues.
Also, what is the temperature in her tank? Bettas do well at temps between 76-84F. (Approximately 79F is considered "ideal.")
I recommend using Epsom salt (starting with a dosage of 1 teaspoon per gal), and getting her water temp to about 78-79F.
Epsom salt can be purchased at pharmacies and supermarkets. Look for 100% pure magnesium sulfate. (No dyes, additives, or perfumes.)
Add 1 teaspoon of Epsom salt into a gallon of conditioned water that is the same temp as her current tank water. Stir or shake until the salt is completely dissolved.
Is she still in her plastic cup? Or do you have her in a tank? Let us know where she's located, and we can give you directions on how to acclimate her to the Epsom salt solution. You'll want to do this slowly, so that you don't shock her system.
As Corsica mentioned, you can also try garlic, fasting and Indian Almond Leaves. (If you don't have Indian Almond Leaves, you may be able to use Oak Leaves. If you want to try this, let us know.)