I agree that it sounds like the cause of a diet change, which can be something as simple as different hay. If you're local feed store has started buying hay from a different source than they were before, thus having different levels of nutrients than before. Taking her off her grain would also have made a difference. I know of some companies that you can send a hair sample too, and they will test and see what vitamins and minerals ect., your horse has at what levels, and what needs to be changed, as well as testing hay as well. I don't know how expensive it is, but you may want to look into that, then you know exactly what it is that your horse is missing, or is getting too much of.