How about:
"Mum, I did indeed put these matters to Jehovah, and he clearly guided me to follow the examples of ones in the bible book of Revelation who were told to "get out from amongst her, and quit touching the unclean things, so we don't share in the sins of an organization"
Too strong??
Or
"Mum, thanks for your sudden interest in my life. As you know, I take very seriously the Bible's command to "test the inspired expression" and to imitate the Boreans who "carefully examined the things taught" so as to please God. I put all these concerns to Jehovah and he has clearly shown me to step back from the organization while a cleansing takes place. I know that you would never want me to go contrary to his leadings, so I appreciate you respecting this process. As always, I love you. Your son"