Just to offer a different take on it. It may not be related to transition at all.
The subconscious uses imagery to portray emotion. And the stronger your association with something, the stronger that emotion can be. It's like the mind's way of pushing your buttons and getting you to think about stuff, even though you may not immediately know what it is you're supposed to be thinking about.
In this case the dream could be about feeling out of place with something in general. Not transition related. Some other part of your life. The change in clothing could simply mean that in some part of your life you feel like you have to be someone you aren't. Or feel like you have to put on a front for something, or someone. Even though it's not who you are. And since it's something you have a strong association with, that was the imagery your mind decided to use, to illicit an emotional response. Maybe as a warning signal that it's maybe not a good idea for you to be someone other than yourself. And part of you knows it.
*big hug* Nightmares aren't nice, whatever they consist of. I'm sorry you had to go through that.