Headache on one side of head is usually due to Migraine, particularly if it is throbbing and associated with nausea or vomiting. Although, Headaches have many causes, common ones are are due to Stress, eye strain, sinus blockage and Migraine. Very rarely it could be due to something significant like a stroke or tumor. Take mild anelgesics with Antacid and rest. If the headache is progressively increasing or associated with vomiting or double vision, you need to consult a Neuro Specialist and possibly an MRI of brain.