I think the only way a cough can be a reflex is if you have something stuck in your throat/airway. Hitting a wall would not trigger a cough reflex, I shouldn't think.
She coughed as she hit the wall, hitting the wall made her cough, a cough escaped her as she smashed into the wall... are some possibilities.
And as to the use of "knee-jerk reaction" - usually, this phrase is used to refer to things that aren't actually reflexes, but are like them, i.e. actions that trigger a certain response, every time.
'Every time there is a new discovery relating to human biochemistry, the knee-jerk reaction of Science News has always been: "This may lead to the development of a new drug," as in "Dopamine's role linked to location"'http://idioms.thefreedictionary.com/knee-jerk...