I'm sure i've heard one or two people report this on the ehealthforum before. I dont think your wife's got anything sharp stashed in there though, heh.
It sounds like the skin on the head may be too tight..? What could cause this, I don't know. Maybe using the wrong soap, or irritation of another kind. To be honest i'm just guessing though, I think if it persists, it's one for the doctor
hope it clears up for you