Don't worry yourself, that is fat. Lots of mares store fat down there. We used to joke about the shapes the fat would take on various mares - with one it felt more oblong and we said when she needed to go on a diet it was because she was getting her "banana" back. We'd joke that other mares were getting an "orange" under there. I'm not sure why but we always related them to fruit.
My current mare only gets apricots - she puts most of her fat elsewhere.
Your idea of cutting back on her feed is a good one.