It sounds like you had a medical eye exam. Even if that was normal, your trouble focusing may be because your eyes are having trouble working together. Only a Vision Efficiency Evaluation (VEE) can pick that up. If your regular eye doctor doesn't do those, you can find one who does here:
http://www.braininjuries.org or
http://www.nora.cc
I had a completely normal medical eye exam but significant problems on my VEE and needed vision therapy to help my eyes work better together. After ten weeks of this, my headaches decreased significantly.