Hi,
I don't know if your T has actually told you about reading your posts; if that was true he would need to know your username (have the correct one to) and have an account to look at your profile to see other threads/posts you'd made; if he wanted to see past posts.
I know I used to keep a friends only online diary (where I knew each friend and had them on FB) so my diary entries were secret. Sometimes I would post things on there and the next week T would say something I had written in this diary. It made me paranoid! I realised though in the end, T was picking up on my unconscious and discussing it with me the next session, so I was already putting out my thoughts in the universe, as well as her room. Sometimes it happens when I write in my paper diary (and I know she isn't reading that as its hidden at home!)
I hope it gets sorted xxxxx