Nutrition maybe? What's your horse eating? If she's getting sweet feed or lots of grain I'd check into that and do a little research. But also many horses will get, "spring founder", simply out on the pasture grasses because they are just standing there gorging themselves all day like a cow.
Nutrition is still something new to me, but it may be something worth checking into.
Truth passes through three stages. First it is ridiculed. Second it is violently opposed. Third it is accepted as self evident.