30 weeks and barely able to function

Ok so first let me start by saying I have a very stressful and demanding job. When I go to work in the morning, I don't actually know if I'm coming home on time or working 8 hours over. I also am on call at least one night, but usually multiple nights a week and some weekends. I've also recently been threatened to be fired due to the low productivity I've had over the last couple months (due to us being so short staffed and being freakin pregnant and not feeling well). So for 3 weeks after that I busted my ass everyday and worked late to try to get caught up. The stress and exhaustion caught up with me and I'm now having some sort of cardiac episodes... starting last week I noticed I was having shortness of breath, racing heart, elevated blood pressure, severe chest pain and it would come on suddenly. I eventually went to the ER but they couldn't tell me what was wrong. So now I'm wearing a heart monitor to see if they can figure out what's going on. But the work stress is not easing up, even with my medical issues. We are so short staff, they can't pull me out of new work assignments until right before my due date, which will give me no time to finish all my assignments prior to maternity leave. They also will not take me off the on call schedule until right before I leave. The whole situation is stressing me out even more. They are expecting me to give 150% Of my normal effort to meet their demands in these last 8 or 9 weeks when I'm physically only able to give about 50% right now. And I can't afford to go on leave early because I have a limited amount of paid leave. What can I do? The stress of my job is gonna make me lose my mind, have heart attack, or cause this baby to come early...