This is as, previously mentioned, perfectly plausible. Harry injected her with something immediately prior to her death. The potterverse has vampires. Timeturning + a bit of stealthy substitution, and she may have died under the correct circumstances for rising undead. Given chapter 93, it would have to be the work of Snape, as the other players are all ruled out for time turning to the correct period, and making a shot of vampire blood look like an oxygen potion would not exactly stretch his abilities. Or he could have faked her demise with more conventional alchemy, but that would require him to either brew the necessary potion in <4 hours, or to just have it prepared already on general principles. (for faking his own death in extremis? Just general "Ways I could have saved Lily" brooding?)
The main thing "vamping" has going for it is that vampires do die and come back, so that plan does not involve faking the death, and locating some vampire blood quickly is presumably simple.