I do wonder how much of this is self-fulfilling atrophy. You're tired and sick, so you stay in bed. Then you get weak, making you more tired. Hopefully she can turn things around, regardless of underlying causes.
It sounds like for cases like hers, trying and pushing yourself to exercise only makes you weaker, not stronger. If you're curious, her channel has a number of videos about this.
Yeah, she was so active and productive and engaging as a youtube content creator. She is one of my favorites! So it's just stunning to see how profound whatever is happening for her is. She went from bright and engaging to basically a state of profound depression (I mean this in a physical sense.)
Hopefully she finds out the whole picture some day, makes a recovery, and will do an extensive retrospective.