Hello, Welcome to the forum. You have not mentioned about the medications you had been taking for GIT infection ( it can be a side effect of the medicines taken ). However, ensure that you are not dehydrated. Take orally re-hydrating fluids to stabilize the electrolyte level of your body. Take soups, juice, pure hygienic water. Keep your bowels clean, avoid constipation. Constipation may be a cause of headache ( patients have such symptoms ). Eat fiber rich food. Sleep well. You should have an eye check-up to ensure your vision is fine. Check out your blood pressure. Hypotension can be a cause of severe headache at the back of your head. The other causes can be cervical vertebral stiffness, you may need X-Ray cervical vertebra to rule out any cervical disc problem. Migraine can be a cause, inflammation of the occipital nerve, any past injury at the back of the head, tumors, dengue fever, sleep apnoea can be the other symptoms. Consult a general physician ( if symptoms still persist ) after you have confirmed that your eye site is fine and taken other precautions explained above. You need to be clinically assessed and get few investigations done. You will be referred to a neurologist if the diagnosis and investigation reports are serious. Alert your doctor with any history od allergy, past illness or injury. Have a nice day, take care.