The soul passes through a realm, Limbo, Purgatory, Yomi before it passes on to its final destination, be it rebirth, elysium, or whatever hells fill out your cosmology. The resurrection spell has snatched the soul in mid stride from that inbetween place, and drawn it back into the living world. Installed in a new body, or its old, healed body, the fact remains that the soul had begun its progression of the soul cycle, and was interrupted.
The soul returned with fragments of visions and memories of that inbetween place. Now, the sheet that seperates the living world from that of the dead, and the other realms is not a constant wall, not of uniform thickness or strength. When the ressurected soul passes near a place that is a repository of other realm essence, it is able to more clearly remember what has occured to it. This creates a sensitivity to such essence, allowing the ressurected to 'see' manifestations of it, and spirits native to such realms. The migraines could be a biological reaction to overstimulation as the brain tries to comprehend that which cannot be comprehended.
This ties back into the shamantic tradition that before a shaman can become a worker of medicine aka Magic, he must cross deaths door, and return. Thus, to gain access to magic, the supplicant must ritually 'kill' themselves ala Flatliners, or Strangeland, and be brought back from the other side.