Is it possible that she has been broody (sitting on eggs)? This would cause her to tear some of her feathers out. Just as likely, she might have mites or lice. Check near her vent, under her wings, and beneath her back and neck feathers, and see if you spot any small moving specks. Mites and lice irritate the skin, and can cause feather loss. Treat them with Sevin powder, or a mite spray like Poutry Protector.
Another possibility is that she has been rubbing her belly against something. I know that when my Columbian Wyandotte dust bathes a lot in wood shavings, she breaks some of her breast feathers. Your's might be doing something similar.
Or, she could be molting. How old is she? This doesn't seem to be as likely though, as chickens molt their head feathers first (the order is head, neck, back, breast, stern, thighs, wings, tail), and you would probably have noticed that.
Other chickens could be picking on her, but I think that this is the least likely reason of all. Normally, chickens peck at the back, neck, and head, not the belly.
Hope I've helped!