It's 100% some form of social anxiety. What I'm wondering about is to what extent it was present from the beginning, and to which an unprocessed traumatic experience induced it or made it worse.
I think something happened that she believes could have been avoided if only she had been more diligent. And now she doesn't know how to take things lightly anymore - not even going to school, and especially not being a magical girl. Someone could die if she went out there in her current state! The only thing she can do that definitely won't cause any harm to anyone... is stay indoors and do nothing at all. Which is what we're observing right now.
Whatever happened must have flown under the radar - which tracks, if we keep in mind that humans can endure insane amounts of mental pressure... for a time. Eventually, they crack. And by the time that happens, it's often not obvious to anyone - even the one suffering - what the actual root cause was.
If it was just social anxiety, I think she would still be able to fight monsters. And if it had always been this present, or this severe, she would never have become a magical girl in the first place. I think we're looking at PTSD, comorbid with a bunch of other issues. And it might get worse before it gets better.