I'm not pregnant or anything, so I feel weird replying to this, but I lost my ability to take naps several years ago. If I take one, I wake up feeling worse than I did before, and I then can't fall asleep at night. Sometimes I do feel sick or headachey after a nap. I then get into this awful cycle where I'm up all night because I slept during the day, and tired all day because I was up all night. It is *just* like what happens when a kid is ready to give up napping, you know? You just have to break the cycle by pushing through a few days of being tired and going to bed earlier, then things tend to even out.
That's not helpful, but it sounds like your body is telling you not to take a nap. I've had to retrain myself to just go to bed earlier or suck it up and be tired. Again, I know, not helpful, I'm sorry. But I think this is one of the "joys" of getting older .