How to apply for FMLA leave before I get fired

Note. I live in the United States, Ohio.
I keep missing work or having to go home sick. I also am making a lot of doctors appointments to deal with these issues which means having to miss more work. It isn't a single health issue that's causing me to miss work but multiple. I went to the doctors today after calling off today. They booked an appointment for a specialist on Friday. The secretary at the doctor's office said I can apply for FMLA leave, something I had not thought of before. They actually booked another appointment 2 weeks later so that I can discuss FMLA leave with my primary doc.

However my boss said she is planning to talk to HR now.

What do I do. I don't want to lose my job. How can I protect myself?