Question....

I was wondering if anyone knew what the bigger red ring around my champion is? It happens randomly during game play. And the ring is solid red, with no transparent filler inside of it. It almost has a diameter close to the size of a turret's ring. I've been told that it is something that happens when someone targets you in PVP, but a few friends and I tried it out on some other friends who were playing against us, and they stated a ring doesn't appear around them when we target them. So I'm super confused as to why this large red ring keeps appearing around me. I mainly have seen the ring appear when I'm playing an ADC champion. Have also witnessed it while playing Morgana.

Thank you for any help!