You must give your employer 30 days’ notice, if you can. If you can’t, give notice as soon as possible. You have to tell your employer your reason that qualifies you for FMLA. This is so your employer knows that you are asking for FMLA leave. Just calling in sick is not good enough.
Your employer can ask for proof of your problem, like certification from your doctor. An employer may contact the doctor to check if the information you gave is true. They do not have to get your permission to do this.
Note: the health condition has to be serious. A condition is serious if it is an “illness, injury, impairment or any physical or mental condition that requires inpatient medical care or continuing treatment by a health care provider."