Laminitis is inflammation of the soft-tissues (called lamina) in the feet. These lamina are what hold the bones, especially the coffin bone) in place in the foot and this bone is what carries all of the weight of the horse to the ground. Founder is a layman's term that is now used to represent a chronic problem caused by the lamina becoming inflammed and no longer being able to hold the coffin bone where it should be so it moves within the hoof to an abnormal position. I would recommend that you go to www.thehorse.com
and check out the webinar (online "seminar") on laminitis. It takes about an hour to watch if you have a decent internet connection. Otherwise just do a search on that website for laminitis or founder and you will come up with all sort of good articles.
If a metabolic condition is present (like insulin resistance) then even a handful of grain can be a problem. And just because she has been on this diet for 4 months doesn't mean that she hasn't been building up to this---any number of things could have "pushed her over the edge" into a visible problem caused by the metabolic condition. Check out this article: www.thehorse.com/pdf/aaep/obese.pdf
And also do a search for "hold the grain" on that website.