Well, for your situation, it is normal to have it. As we know that when you have pinch in your skin, it will have some painful feeling. And then the pain will lead to dilated pupils in your eyes. In other words, you can say pain nerve system can just cause this situation. By the way, high blood pressure and anxiety can also lead to dilated pupils too. You need pay more attention to them.
It seems that you are quite interested in this natural phenomenon, so, I can see you wanna know why this happens. Anyway, as a matter of fact, our eyes and pupils will be dilated when we run into some emergent situation or dangerous situation, or being panicked by something horrible or surprising. So, your pinched skin could give rise to some irritative feelings to your brain, thus causing dilated pupils. It is OK anyway, take it easy.