Fridge Horror realization: She
didn't "recover" magically. She was in a drug addict-like hormonal withdrawal, and proximity to Sou gave her another kick of dopamine and serotonin, so
of course she suddenly feels better.
The screwed-up natural selection of this world made her a neurochemical addict (for the sake of ensuring humans survived x0,000 years ago) and she's
relapsing into her addiction because her own brain is drugging her.