Ask HN: Is someone accessing my private Google Calendar?

Hi HN, I hope one of the readers here has an answer for me.

I was just in my google calendar's settings and saw that under "Share with specific people", my mom is listed, with her email address. I vividly remember seeing this a few months ago and hectically removing her, but now apparently she is back (I don't share my calendar with anyone else).

I clicked on the X to remove her and got the confirmation "Sharing Settings saved". However, after refreshing the site, her email is back in the "list" (it's just her because I don't share my calendar with anyone)!

Is there any way I can ensure my mom doesn't have access to all my private calendar events? I intend to create a new calendar and move my events to that calendar, but perhaps there is an easier way?