I think this may be better off in the criminal law section - I will try to get it there for you. Good luck.
Stephanie O. Joy, Esq., of JoyDisability, is an attorney licensed in New Jersey, but currently practicing federal Social Security Disability law in all 50 states from her PA office. Answers to questions are for general purposes only and do not establish an attorney-client relationship, nor do they constitute legal advice. Rather, if you need representation or legal advise, you need to make direct contact yourself, and inquire. We welcome and respond to all phone calls and emails.
This isn't really something that you can handle with advice from strangers on the Internet. You need a criminal defense attorney who can work with you on this case, whether it's a court-appointed public defender or someone you hired.
If the welfare fraud was committed before you were placed on probation, it would not be a probation violation. However, the judge could deny probation on the new case based on your criminal history.
You also say your girlfriend lied on her application. There is no such thing as a real "expungement" in California. Cases can be dismissed under Penal Code §1203.4 after successful completion of probation, but that just adds some notes to the court files saying the offense was dismissed. There are some advantages to a 1203.4 dismissal, but it must be disclosed on applications for government benefits if specifically requested.
PLEASE READ BEFORE CONTACTING MY OFFICE: My office is located in Chico, Butte County, California. I offer free consultations to potential clients whose cases are in counties within a 100 mile radius of my office. In order to represent someone who lives outside that area, I would have to charge enough to pay for several hours of travel time to each court appearance. If you're outside that area, it will be more cost-efficient to hire an attorney who practices in your community. I do not accept telephone calls regarding my posts on Avvo. If you need more information, please use the "comments" button or post another question. This is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this site, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.
You have two options: hire an attorney asap or wait until you're charged and have the court appoint an attorney. Regardless of what you decide, cease talking about this incident to anyone - friends, relatives, enemies, colleagues, etc - except an attorney in private. You may have already made incriminating statements to the investigators that came to your house but that's water under bridge, from hereon, only speak to an attorney, whether privately retained or court appointed.