Anyone on probation knows that you must cooperate with your probation officer and tell the truth when you do. However, sometimes, especially for sex offense cases, the questions from the probation officer or therapists may be such that you will be admitting to crimes that are not known to the police and you will therefore be incriminating yourself and opening yourself up to new charges.
You have a right under the 5th Amendment to the US. Constitution not to incriminate yourself. However you must actively raise that right during questioning, even during a polygraph examination. If you answer those answers can be used against you.
If asked about crimes that you committed but have not been charged you should tell the probation officer that you cannot answer that particular question because the answer may tend to incriminate you. Of course, you may not lie about any answers as this itself is a violation of probation.
Better still, if you know that the probation officer will ask these types of questions or you are going to take a polygraph test, first consult your lawyer to make sure you know your rights.