So i noticed this happen to me twice already. When i'm drawing something for a long while, the stroke of my pen randomly gets pixelated. I don't know why. I really really need help. In the picture provided, you see my drawing and how the strokes look normal (right) and how the stroke randomly gets (left).
Your layer is set to a Blend Mode of Dissolve. That will result in pixelated edges, it isn't how long you are drawing. Go to the top of the layers panel and set the Blend Mode of the individual layers back to normal.