How Can I Stop My Mom From Accessing My Messages and Photos?

I recently discovered that while I've been away at college, my mom has somehow accessed my phone's Photos and even received my iMessages. This was news to me—I found out through my sister. I've checked and made sure she's not signed into my iCloud account since I bought all my devices myself and pay my own phone bill. I set up my phone with my Apple ID from scratch and never logged into her phone with my ID. The only access I've given her is my location via Find My. I thought I had disabled iCloud Messages and Photos, but it seems like she's still getting my messages. I confronted her about this, but she just brushed it off. How can I permanently prevent this from happening?