Why does handwriting change? In New York magazine it is stated:


  [I]t’s strange that your handwriting changes over time; scientifically speaking, there’s no reason it should. At a certain point, you’ve learned all there is to learn about it: You’ve mastered the fine motor skills, you know the right way to grip a pen, you’ve written enough that you no longer need to think about the physical act of forming letters with your hand. 


But they continue:


  But still, odds are decent that your handwriting right now looks a little bit different than it did five years ago, and will look different again in five more. 


Now I am wondering if handwriting can also change on a smaller time scale, and specifically with regard to emotional states. If yes, I would be interested in what part of the brain controls it.
The brain is an interconnected whole, when we have high activity in one area of the brain other connected parts are affected.

The writting is mostly controlled by the part of the brain that controls movement (lateral and medial precentral, motor cingulate and primary motor cortices, and postcentral gyrus). Research revealed that there is a direct path between amygdala and the parts of the brain that controls movement. Subsequntly writting will be affected when the amygdala is highly active.


  Using diffusion-weighted magnetic resonance imaging and probabilistic
  tractography on a cohort of 40 participants, we provide evidence of a
  structural connection between the amygdala and motor-related areas
  (lateral and medial precentral, motor cingulate and primary motor
  cortices, and postcentral gyrus) in humans. ... a direct
  amygdala-motor pathway might provide a mechanism by which the amygdala
  can influence more complex motor behaviors.


Reference

Grèzes J., Valabrègue R., Gholipour B., Chevallier C., A direct amygdala-motor pathway for emotional displays to influence action: A diffusion tensor imaging study.