Baby Throwing Self Back

Hello...my daughter is 8 months old and has gotten into a habit where when she is upset she throws herself back and slams the back of her head into my chest. She doesn't cry but it hurts my chest so I know it hurts her too. She does this at least 4 times a day and it's so sudden that she does it before I have a chance to avoid it.  Does anyone else's baby do this? Is there anyway I can get her to stop!