I don't know what to do

I'm 34 weeks pregnant and work for CPS. I am due the first weekend of August. Right now in our office we only have 3 investigators so we are working on call shifts ALL. THE. TIME. I'm so stressed and exhausted and worried about how to perform my job duties in this last month of pregnancy.

We work 3 day on call shifts on the weekends. I specifically requested that my July on call weekend be scheduled for early in the month so I'm not on call any time near the end of July, nearing my due date. My boss refused and scheduled me the 2nd to last weekend of July as well as the first weekend. Being on call requires over time, being woken at any time of night, traveling possibly an hour from the nearest hospital, dealing with people who are possibly dangerous... So on and so forth. There's no reason to have me on call that late in pregnancy when the baby could come at any time. But she refused to accommodate my request.

So today I talked to my doctor. My doctor agreed, it's too stressful, I need to rest when I'm not working my regular schedule. So she wrote me a note simply asking to excuse me from being on call this last month of pregnancy. My boss and her boss refuse to accept it. They say I have to have medical documentation for why I cannot perform my work duties. They asked why I'm able to work my normal hours, but I can't work on call... They wanna know how it's any different.

Are you kidding me? Because it's several hours of overtime generally. It's exhausting. It's highly stressful. I've already been to the ER with an anxiety attack once in the last few weeks because of the amount of stress I'm under. They've threatened to fire me because my work productivity has been slipping since I've been so fatigued and stressed. I don't know what to do anymore since they won't even consider giving me a slight accommodation for these last few weeks.