When to tell your job...

I work in retail, usually long shifts, which consist of running around, picking up heavy boxes, pushing carts inside when we don't have a cart attendant which is 6 or 7 hours we don't have one per shift... so I'm always stuck grabbing carts outside in 90 degree weather. my jobs ac isn't the best either so I'm constantly feeling dehydrated no matter how much I drink, and then im stuck holding it for an hour for someone to cover me so i can use the restroom. i had to call out for my Saturday shift, I was extremely nauses. a little back story, the beginning of July I was in the hospital for horrible stomach cramps and pains, I was out of work for 2 days. Drs think it was a misscarrage or some off balance with my hormones causing me more pain then ever. anywho.... my job was on the verge of firing me yet, at that point I had proof and called out for both of my shifts and had drs notes... my boss told me she was going to have a talk with me about me calling out Saturday. like I'm pregnant I can't control anything. I feel fine one day and ran down the next. I know the can fire me for calling out, for anything they want really. however I can't lose my job. I'm just stressed and it's not good for the baby.