It could be down to many things......there is quite a long list and not everything is related to Diabetes or Medication. It could be something as simple as eye strain, stress etc. You maybe need to go and get your eyes tested and see your GP as well to see what reason there is for this. no need to suffer but it needs sorting.
Most people with isolated tension headaches usually avail themselves to over-the-counter pain relievers and anti-inflammatories, or self-treatment such as relaxation and sleep. However, for chronic headaches contact your GP to discuss possible causes and prevention.