Re: Brain tumor?
You need your eyes tested. Even if your vision is "normal" your eye muscles can still be strained. Magnifying glasses with a filter may help.
As far as a tumor goes. It is highly unlikely, typically it is severe headache, nausea, vomiting, and a poor neurological test. Seeing how you have been to the Dr. I would hope they did a basic test on you.
Honestly eyemuscle fatigue sounds much more likely.