If you notice it happening mainly from your peripheral vision then its definitely a hallucination. Other than that im not sure. Best thing you can do for yourself though is try not to stress about your experience with it. And also possibly bring it up with your doctor next time. I dunno.
Also, when i hallucinate occasionally these days. I often saw crazy things like people with snake-like pupils in their eyes. Definitely wasnt real but i was freaking out about it a lot at the time.