Is it normal that my brain hurts or feels full when i think too much?

Muscle tension: We humans tend to wrinkle our brows, tighten our necks and narrow our eyes when thinking, then the muscles get tighter, making the head feel "full". It's like pulling tight cap down onto the head for hours. Relax the muscles, take a break, stop reading, move around. Roll your shoulders and head, take a power nap, anything to unlock the muscles.