u/Dead_general_rhea

I need advice on a concerning situation. I left a previous workplace earlier this year. After my departure, I stayed in touch with two former colleagues through personal messaging. A junior colleague later received a vague negative performance review from someone who hadn't directly worked with her. When she asked for specific examples before signing, they refused to provide any and ultimately let her go.
Understandably upset, she shared her feelings with us privately on WhatsApp. Recently, a current manager there created a group chat with me (long since left), the former colleague (who was let go), and one current employee—then confronted us about supposedly speaking negatively, quoting from our private conversations verbatim.
Here's what concerns me most: these were personal messages on our own devices and accounts. I don't understand how they were accessed. I'm worried my phone or accounts may be compromised.
What steps should I take to secure my devices and accounts? And is there anything else I should be doing from a legal or safety standpoint?