If Employee is aware of your Fmla leave & Dr Certification was turned in: how soon can you leave work?

Im a California State employee, my parent got sick & lives out of the area. Late April Early May we learned heart surgery is needed. My employer was notified at U was. We had been waiting for a surgery date. Early July we had been given a date for the later part in July. My employer asked about my plans as I asked for FMLA. She was aware of the #of weeks i would be off but I didn't have Fmla Dr Certification yet. Now i have it and turned it in and Now its time for my parent release from hospital & my Supervisor says I cant leave, I need to report to work until my fmla paper work is APPROVED. Mind you i have just about all the time on the books to cover it. Is this correct? No Union Rep was available. If Employeer is informed with Drs Certificate when are you able to Leave Work? Per Drs Certificate I should have been off That day but I still worked That day, Then for Supvr to tell me I need to return to work the following week as normal.