You do not HAVE to apply for FMLA. If you, your medical condition, and your employer all qualify for FMLA protection (and this appears to be the case) then your employer MUST apply it. They don't have to tell you, they don't have to ask you to fill out special paperwork, they very simply have no choice but to provide FMLA protection. Along with the protection for your job, the law also specifically allows the employer to make any time off under it UNPAID. You are taking FMLA leave whether you like it or not. Without FMLA, your employer could simply fire you for not being able to work a full day.