It could be your eyes, poor/ failing eyesight or other types of problems with your eyes can cause headaches.
Also teeth and the alignment of your jaw. something simple like you've had a tooth out
and now you chew on the opposite side to which you used to can cause things like that.
Also as has been mentioned the position of your head / neck alignment.
Something simple such as they gave you a different chair to use at work means your posture is now different
throwing the alignment of neck/ head out causing strain.
You could be studying for a new course and putting extra hours in to that.
Stress / lack of sleep / poor nutrition.
Or the way you walk your dogs.
Headaches are funny things they can be caused by so many things.
The headache is simply the symptom not necessarily the cause.
While you hate going to the doctors they are your best bet.
If a headache doesn't clear up after a couple of days using paracetamol / ibuprofen
then it's not "just a headache" and you need to go see the doc'