First, you should change your password. It may be that Wisconsin has a crime for impersonating someone else. you should consider complaining to the police, but the bigger question might be how you spend 10 days in jail based on an email. You should consult with a criminal defense lawyer in your area about the probation violation as well. This response is for informational purposes only and is not offered as legal advice.
Using someone else's email violates the terms of service of many service providers. Using that email without the person's authorization can violate the federal Computer Fraud and Abuse Act and potentially state laws as well (you would need to consult with a Wisconsin for that). More importantly, you may have a claim against the ex-girlfriend for lying to your probation officer and getting you locked up.
I agree that you should consult with a criminal law attorney to fight the probation violation and to see if you can bring an action against her.
One last thing - this is why you should never give out your password, make sure no one can easily guess it, and change it after you break up with a person if he/she knows it.
[This communication is intended as general information and not specific legal advice, and this communication does not create an attorney-client relationship.]