Laid off while on maternity leave

After 10 years of employment and a flawless record, I have been laid off while I am on maternity leave. I went out of work at 6 months pregnant due to back pain. In my last month of pregnancy, I got a call letting me know that the entire department was laid off and I was going to be when I returned from maternity leave. However as time has passed, this is not actually the case. There are 11 people who are staying to manage the department and one of the people staying is in my office performing my job duties but at a lower title (manager). I am still on maternity leave so I am not officially laid off until I "return" from maternity leave however I am told I will be. My personal items have all been packed up and my phone has been disconnected at the office. My HR department had also "miscalculated" my severance leaving me short 10 weeks that I was owed. I pressed them on several occasions to inform me how my severance was calculated and only when I demanded, did they concede that they made a mistake. I feel that my job is discriminating against me because I was out on maternity leave. Is there a rule that if you go out on disability that they have to have your job waiting for you upon return?

I live in California.

The Family and Medical Leave Act allows up to 12 workweeks of leave in any 12 month period for eligible employers and eligible employees. If both your employer and you as an employee meet this definition, 29 CFR 825.214 is clear on what the returning employee is entitled to:

(a) On return from FMLA leave, an employee is entitled to be returned to the same position the employee held when leave commenced, or to an equivalent position with equivalent benefits, pay, and other terms and conditions of employment. An employee is entitled to such reinstatement even if the employee has been replaced or his or her position has been restructured to accommodate the employee's absence.
(b) If the employee is unable to perform an essential function of the position because of a physical or mental condition, including the continuation of a serious health condition, the employee has no right to restoration to another position under the FMLA. However, the employer's obligations may be governed by the Americans with Disabilities Act (ADA).

This is by no means a guarantee you'll get your job back, as you may have exceeded the timeframe. Also, if you live in an "at will" employer state, your employer can both hire and terminate employees at their own discretion. Ask for a written notification of your termination and then contact an attorney. (It's up to you whether or not you decide to wait for the actual letter to arrive before you talk to an attorney ..you may never see it.)