UPDATE: OK it's roseola! a.k.a. 假疹. Alarming as the fever can get pretty high, but apparently it's harmless and the only treatment is to let it run its course. She really had a hard time when the rashes erupted — was up the whole night and kept clinging to me. I couldn't put her down at all. We could only get some sleep around 4am. When dawn came we bundled to the hospital. Thank goodness she had some relief with Calamine...
But the poor thing's lost precious weight... A sick baby is a sad sight to behold.