Recently, I've been told that someone who I knows therapist has been talking bad / fake things about me to them. Is that Illegal? I've not signed any paperwork or anything.
It would only be illegal, if you had a therapy session with them and they then talked about you to others afterwards. That would be breaking their therapists confidentiality clause.
Therapists/doctors etc can only break that rule if they are sure you would endanger yourself (suicidal) or others or if you had admitted (to them) about causing a crime (i.e. murder).