Wiggle your big toe.

It's called ghost limb syndrome or something.

I think it's because the nerve endings there all still work for the most part, and because they're healing after having the leg removed I guess they're irritated so they send the brain the same signal they do when it's itchy.

Just my theory, I know nothing about this stuff.

you're close. the rest of the nerves are still there, and the brain still subconsciouly thinks the limb exists, even though it doesn't.