Take your pick! Brain tumor, sinusitis, multi-infarct dementia, transient ischemic attack, temporal arteritis, subarachnoid hemorrhage, lead poisoning, paresthesia, tmj, abcessed tooth or teeth, cracked tooth, eye strain, etc. In short, do not self diagnose, seek professional help as pain is usually a sign your body needs help. Most importantly, make sure the final diagnosis makes sense to you!
It may. Tension headaches occur when we are stressed. Muscles involved in tension headaches are located on the forehead to the temples and all the way over the top of the head to the base of the skull. If you are truly straining to see, you can develop a tension headache. Pain in the temples, especially to touch, could be a sign of a serious condition, so see an eye doctor.
It can. There are many different things that can cause headaches so the first thing to do is to make sure that the eye strain is the reason. Muscles inside the eyes contract when near work (like reading) is being performed. Prolonged periods of such contraction can lead to eye strain and headaches which can radiate to the temples; glasses can support the muscles so they don't have to work as hard.