Ketosis after lap band surgery is quite obvious and your case is slightly different. I know two people who had done this surgery but never encountered any kind of migraine during ketosis. I believe the diet you are consuming is not in accordance with what a lap band patient should consume. Another reason can be that your exams are round the corner, so you are stressed up and this in turn is causing pain in your head. Relax for now because any kind of stress can become harmful for you. Take some pain killers and see if the pain gets subsidised or not. If it continues, my best advise would be to consult your surgeon.