How to take leave early?

Just wondering because I'm 35 weeks and planned on going off work at 37 weeks. My work does FMLA and Short-term disability and won't allow employees to miss more than 3 days without a doctors note. My doctor refuses to write me off early since I've had zero medical complications. I teach Head Start and am on my feet 8 hours a day doing a lot of bending, squatting, and lifting. I get home and I'm in so much pain all I can do is lay in bed. Something has got to give!