I'm about 31 weeks and currently work 10 hour days and 2 hours travel. My work says I need a…

I'm about 31 weeks and currently work 10 hour days and 2 hours travel. My work says I need a doctor's note to reduce my hours. I have expressed to my doctor that I am beyond exhausted and extremely uncomfortable trying to get through the day. Sitting at a desk (even taking breaks) is giving horrible back pain. She says she won't sign a note because there's no “medical reason”. Im not asking to be completely be taken out of work…just to be able to lighten my load and have what little energy is left. I feel like she isn't listening to my concerns really and I don't know what to do. I never thought I would be getting such a hard time from someone who is supposed to be an advocate for the pregnancy. Advice?