Congrats!!
When I was pregnant with J I had to ask the teachers next door and teachers aides if they were available to cover my room for a few minutes while I took bathroom breaks. I also had a taller bar stool type chair and would sit occasionally when teaching if I could. I also would park myself at my reading table to check work or work with individuals/small groups when I would normally walk around and meet kids at their desks.
I don't think you need to tell your principal until you are ready to tell.

If you think she's becoming upset because she sees you sitting, you could tell her and ask her not to share with others. I think she'll understand!