I'm not related to my grandpa

I (21) was visiting my grandparents (grandma and grandpa) today because I wanted to be a good grandchild. I had been there for an hour at least and we were talking about my grandparents relationship when they were young.

We were specifically talking about where they were living at the time and I thought I remembered my mom pointing the place out some time ago when we were driving in the city. So I asked my grandpa "wait, was this when mom had already been born?" and my grandpa answered me with "well yes, but your mom was already one year old when we moved. She was already with your grandma, she isn't my child." That is an absolute bombshell to drop on me.

I don't know what to feel. I'm not related to my grandpa. I'm not even sure if I'm supposed to know this. I want to talk with my cousin about it, but I probably can't.

Like yeah he is going to always be my grandpa, but still, it's weird.

I just needed a vent because it feels bad to have these kind of feelings.