She has a GI infection. It's either parasite in nature or bacterial. It's hard to tell as the symptoms are identical.
That looks like a 1 gallon heated bowl and unfortunately bettas don't do well in those. Unless you're in a hot area and your house stays above 80 degrees year round, she's freezing and that slows down metabolism and immune system and leads to infections like these. Also, the size makes it difficult to keep ammonia levels under control, which further breaks down their immune system causing them to get sick more easily. Can you fill this out please: http://www.bettafish.com/showthread.php?t=49233
You need to get her in some epsom salts. 1/4-1tsp per gallon, predissolve in some bowl water and then added in.
You're also going to need a good antibiotic. Since she's eating, I suggest feeding the meds. Get some good soaking pellets like Hikari, garlic guard and metronidazole by seachem. Put a small amount of garlic guard in a cup along with the pellets and let them soak 10-15minutes and feed 2 to 3 twice a day for a minimum of 2 weeks or up to a month and at least 1 week past when it seems she is totally in the clear. If she isn't better after a week on this I would switch to Kanaplex directly in the water, not fed.