It is really hard to tell what cause grainy vision without eye examination. Because there are many factors that may cause it. For example, the internal problems in the eyes can contribute to grainy vision. For example, if you suffered from blockage of a retinal artery or vein, it is probably cause symptoms like grainy vision. So, you need consult an eye doctor if you want to a good treatments for your eyes.
Relax,this is quite normal and u really don't need to concern too much about this.I have also experienced this symptom once in a while and I can deeply understand what u feel now.I once went to my eye doctor for the reason who told me that this was very normal and common and what I should do is to keep a balanced life so that everything will still be fine.