The FMLA is a federal law. If your FMLA rights have been violated, you may file a complaint with the federal Department of Labor.
www.dol.gov
Yes, your employer absolutely may require you to explain why you're absent and/or why you are taking FMLA. Among other reasons, they have an obligation to determine if the absences qualify for FMLA. There is also a FMLA certification form they may require you and your doctor to complete which includes a diagnosis/reason for taking FMLA.