Can someone with medical knowledge attempt to compile her symptoms and come up with a possible diagnosis?
Sudden onset of feeling like shit, fever, doctors not prescribing antibiotics the first visit, and a sudden recovery is textbook virus. She was probably given antibiotics on the second visit to shut her up, not because she needed them. It didn't present as cold/flu or gastro, so probably just an opportunistic infection that burned itself out without any special manifestations.
TL : DR; she's unhealthy to the point that something that an average person's immune system could shrug off is enough to make her sick.