Dull, achy pain at the forehead and behind the eyes can be a tension type of headache.
This headache, normally begins in the middle of the day.
Sinus headache is usually associate with other symptoms of sinus infection (nasal obstruction, nasal discharge), and are worse in the mornings, and, when changing head position (leaning forward).
However, there are some cases of headaches related to chronic sinusitis without other upper respiratory symptoms.
It will be a good idea to get checked for your vision, too.

I was at a friends house a few weeks back where there 10yr old boy complained of a head ache. His mum reached for some pain relief & i told them to wait, told the young boy to get aglass of filtered water, which he did, then i put a half a tea spoon full of unprocessed natural salt in the glass & stirred it untill all the salt had disolved. I told the boy to drink the water slowly. 10 minutes later the boy reported exclaimedly that his headaches were gone & he felt really well.The short version to this story is that dehydration & mineral loss can present it'sself as a variety of different everyday symptoms, we need to listen to our bodies for self help as the best preventative!