Founder is where the Lamina in the hoof inflame causing pain on the feet, and can progress into laminates (where the coffin bone sinks and rotates) causing extreme pain and can be fatal (EX coffin bone falling out of the bottom of the hoof). I would get a vet on this asap. Founder dose not cause weight loss and a weight change that quick with stiffness needs to be taken seriously.
I have always thought it was the other way around;
Laminitus is where the laminae become inflamed, and allowed to progress, it leads to the condition called "founder" , where the coffin bone rotates since it is not longer held firmly by the laminae (they loosen and literally die).