Sounds more like a pain issue and not EPSM. A simple test I do with a suspect EPSM case is to send them through a gate, have them pivot and face me. Is the movement fluid or is it herky jerky on the backend? Is there muscle atrophy and asymetry through the loin and rump? Next, back her up on the lead. Fluid or reluctant? No harm in putting her on an EPSM diet but you've got to stick with it for at least a month. An EPSM diet isn't about a high sugar diet, it's about muscles that can't use glucose as a fuel source and are starved for energy.
Take a serious look at your saddle.