Work, pregnancy, and being sick

I called work today because I have a fever and to say I might not be able to make it in. I've never called off my entire pregnancy, not even the last time I was sick. The last two weeks, they have really been over working me, 10-12 hour shifts, 6 days a week. I'm up on my feet everyday, doing things that most doctors would disapprove of right away. Anyway, Thursday when I worked, the manager told me I could call her if my sickness got worse and I could trade shifts with someone, which the other employee agree with. I fought through it Friday and Saturday night, closing the store alone. So this morning, my body is aching, my head is pounding, my chest feels like someone is sitting on it. I feel terrible, and have a fever. I called in this morning only to be told I need a doctor's note so I can come back to work and for today, even though I traded shifts with someone else and technically didn't call off. I was going to the urgent care anyway, but the outcome might be one that hurts them more than me. Idk, I just wanted to get other inputs on the matter.