Same thing happened to me in a Pitts only I was wearing a helmet and didn't feel it. But I wrenched my shoulder and was in pain for weeks afterward. 

  I wasn't flying it at that point so was taken by surprise.  Took him by surprise though, when I fell out of a roll into a spin. 
 
 Aerobatics is like being beaten up sometimes, sore heads and pulled muscles. 
 
 Sadly only flying boring old 172's and 150's lately. sigh!!!!
PERK THE RV8!!