I suspect its the spring grass, they act as if they have had too many blue smarties sometimes after eating. Just keep an eye on her, she could also be in season or coming up, they can feel sensative around the girth area etc. Obviously if he keeps happening then, check out the obvious of tack etc. Just watch the spring grass, as it is well known to bring on the old laminitus, I am at the moment strip grazing so they are in the main field during the day but, limited at night, but thats because I am worried about weight gain and laminitus, got caught out two years ago when our mare came down with it after 3 weeks of being on the spring grass. good luck