Thinking about leaving work early...

Well, since I am not due until July, I always thought that I would not have a problem making it to the end of the school year, but lately being on my feet all day has really been taking a toll on me. By the end of the day, my legs and feet are so swollen and discolored (redish/purple) that I am miserable. I went to the doctor today and she told me that everything else looks good (blood pressure, weight gain, protein levels, etc), so I don't really need to stop working, but if I am uncomfortable and miserable, she will write me out of work whenever I feel like it is too much for me. I almost wish that she would have said "You need to stop working on X day, or at X amount of weeks" because now the decision is pretty much in my hands and I am feeling so guilty about the possibility of not finishing out the school year and being there to prepare my students for their exams. Part of me feels like it is not really necessary to stop working so soon, but then I also feel like I'm not really sure I'm going to be able to make it until the end of the school year.