Sounds to me like you're dehydrated or you don't move in your sleep.

When you sleep and something I.E. a seam of a blanket is pressed up against your arm for a while you're going to wake up with a mark........ It might burn because it's been pressed up against the blanket for so long. This happens to me, has for as long as I can remember....

I think you over analyze a little too much, just because your arm itches doesn't mean you have a paralyzing spider bite, most likely it was just a harmless bug.