It's the same reason I feel my third eye ache through my forehead when I see a Yehubor. Even a mixed one. Their energy is very contrary to ours and alien.
As HP HoodedCobra said, it's possibly related to disgust. When I'm near obviously Yehuborim people I feel a shudder of disgust. It's not so much pain as it is dread, disgust, and uneasiness. Which is a normal response to anything that is unnatural, unsafe, or a threat. Everyone is different though so these feelings likely manifest differently for us all.