i just posted about something very similar to this. i am 18 and have been having headaches like this for a couple years now. when i go to sit or stand or move my head suddenly there is sharp pain. plus the dizziness. and i feel disoriented. i hope someone responds to these. and i sure hope its not what the above person is talking about!
I am not medically trained however I recommend you schedule an appointment with a neurosurgeon as soon as possible. If you do not know of any and are not able to have a family doctor to assist you in obtaining an appointment, go to the emergency room.
I have a LP Shunt to relieve excess cerebal-spinal fluid/pressure. The valve for the shunt failed causing an overdrainage or Intracranial Hypotension. The symptons I had were very similar to what you describe (although I never had memory loss) and I was told this type of headache is often the tell-tale sign.
Regardless of what the cause is, your body is trying to tell you something is wrong. Get checked by a specialist right away!
oh yeah i forgot my vision gets blurry as well at times