Why Did I Lose Hair After Bumping My Head?

I bumped the back of my head about two months ago. Since then I still have a bump and a bald spot the size of a band aid where the bump is. What could cause this?

You could have put that hair into its sleep phase and if that is the case, it will come back in 6 months from the time the injury occurred. If you cut the scalp, a scar could form that may not have hair in it. You should see a dermatologist if the bump is persistent, as 2 months seems awfully long for a mild head bump. Usually swelling will go away within days or even as long as a couple weeks… but months?