When throwing up what cause your head to start itching?

That is a very interesting thing to happen!  Does your scalp develop any hives?  Is this a one-time occurrence or does it happen every time you vomit?  It could be from the stress of vomiting that this occurs.  Stress can bring on hives or urticaria, so maybe it can be causing your scalp to itch.  Let me know more about your problem, and I can see what I can do to help.  I look forward to hearing about your situation.