Mom's mobile phone often receives my short messages inexplicably, and I have never sent them to her.

This is anti-theft protection. Your mother or father used your mobile phone.

For example, the mobile phone 360 has this function, which can remotely lock the mobile phone (lock the lost mobile phone), remotely ring (let the lost mobile phone ring), remotely back up (back up the lost mobile phone data), and send a short message to a safe mobile phone number after changing the SIM card (you can set the content yourself).

You log in to the security software of your mobile phone, and then ask your parents if they have set the anti-theft protection function.