I have in fact been 'sucker punched" in fights outside the ring and received punches to the back of the head inside the ring..which is illegal. Without going into a lot of medical terms and reasons for not allowing punches in the back of the head, the short answer to your question is : Yes, you can be knocked out or will feel dizzy...tha'ts why it's the preferred punch of a street fighter. Yes, your neck will feel very stiff...even to the point it hurts to turn your head. You might even experience headache. My advice is to lay off any head blows for the next couple of weeks. My best advice is to find another sparring partner...this guy has no idea of what is expected of a sparring partner and sounds like someone who could get out of control real fast.