I have had MS for a couples of years now and I am under DMT (Tecfedira). I have very mild symptoms (balance issues and fatigue) and have been having a pretty normal life. Over the past few weeks, I seem to have sudden episodes where I feel as if someone suddenly just removes the batteries. All of a sudden, my head feels heavy, everything is a struggle, I can’t think straight and if I wait for too long I struggle to walk. I can then sleep for a couple of hours or rest and everything comes back to life.
Its worth going to your GP and being tested for thyroid deficiency. that too causes the symptoms. As said above fatigue is very very real and feel like you have totally hit a brick wall of exhaustion. Basically, give in and rest.
As well as having your thyroid checked (underactive and overactive thyroid can cause fatigue), you could get your vitamin levels checked too, vitamin B12 and D especially.