Random person received my video?

I was sending pictures and videos back and forth with my grandfather when all of a sudden, a random number group texted us and said "who is this." I had no idea where they had gotten my number and asked. They said that I had sent them a video in a group chat. I've never seen this number and looked back through my text messages to make sure I didn't send a text to this random number by accident. I even face timed with my grandfather and checked his text messages to make sure he didn't text this person by mistake. I'm at a total lost and really don't want anymore of my personal things to be sent to strangers. Someone please help! How did this happen?!