When to stop working.

How do you know when it's best to stop working? I am a geriatric nurse and it hasn't been too difficult yet however the first half of my shift I am the only nurse and I notice by the end of the night I have so much pressure down below, and I have extreme fatigue to the point the bottoms of my feet BURN, and I could cry I'm so tired. I know it's because I'm going on 33 weeks and there is a lot of weight down there but I'm trying to push as long as I can due to my husband will be the only income coming in once I'm out.. any advice on what to watch for or when to tell myself I need to stop? I do not want to over due it, I have to have a cesarean and I cannot go in to labor due to a shifted uterus and he cannot come down the birth canal or he will get stuck like my daughter did. just looking for advice. thanks!