Nervousness triggered by drawing dots

Hey, this question might sound weird but I'm really interested in knowing what you guys think.

So I was sitting in front of my desk, studying for an exam and thinking how to solve a problem.

I started drawing a circle on a blank page and decided to give it some depth with shadows which I began by drawing little dots where the shadow would be darker. I started dotting the little circle but I soon realized that the further I went the more I was feeling anxious, almost nauseous up to the point where I couldn't continue. As soon as I stopped, the feeling went away and it would only come back if I continued

Now. I am interested in knowing how and why my brain reacts to something so simple as drawing dots so negatively, so I thought someone here might give me an answer or maybe someone experienced something similar.

I am not an anxious person and never usually feel anxious out of place. I would rule the fact that I was studying for an exam as I have procastinated before without the same outcome. A little search on Google showed that a lot of people feel in a similar way when looking at holes but it doesn't bother me particularly.

Any ideas ?