An itch you can't scratch

I just got back from a trip to the dentist. Half my face is numb. I keep biting my tongue though I have no idea how badly I've bitten it because I can't feel anything. And I'm clenching my teeth because I can't feel half of them either. But that's not the worst part. The worst part is this itch on the numb part of my chin that won't go away because I can't feel myself scratching it. Why can I feel an itch when I can't feel anything else? That just isn't fair.