Can somebody explain me the science behind having a headache when reading when y!

A headache only during reading following a concussion may be related to problems with convergence or focus in the eyes, or it might be related to having the head in a certain position for long periods. This may or may not actually be related to the concussion. Concussion itself can produce very persistent headaches that are uncorrelated with reading activity. Concussions can vary a lot in their symptoms from one case to another (even in a single individual, if he has suffered more than one concussion).

The microbiology and neurology behind this is not well understood. It's not possible to usefully examine brain tissue in a living person to see exactly what damage has been done, and even the normal functioning of the brain is so poorly understood that it would be hard to see if anything were amiss, anyway.

If you've had a concussion and you now experience headaches when reading, tell your doctor about it the next time you see him, but realize that he may not be able to provide an answer as to why you are getting the headings. He might be able to prescribe some effective pain relief for them if they warrant treatment, however.