Abby, Has anyone ever imaged your hip area? What about your spine and/or brain? Don't assume it's the MG, though it could be.
Yeah, I have core weakness. You can strengthen those muscles, even with MG. I would highly suggest an experienced PT to help you though. My neuro has PT's in their office who understand NM diseases. That really helps. We worked together to figure out what would work for me.
When you described this new weakness, it reminded me of a puppet when they are flopped over.

MG is a wicked puppet master. Hope you find a way to make it better.
Annie