It sounds like you have quite a talent with animals to me
And the general rule about stomach problems is lighter food, so it sounds like a good idea (I have no idea about the details of the horses' diet, but as far as I understand you talk about giving her lighter food, right?)
Could it be food poisoning from something she ate?
By the way have you heard about shielding? It could help you to not get too affected when you detect pain.