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To start with, I have a phobia of looking good. As in, I feel physically sick when I have to put "nice", "good-looking" clothes on, especially very girly ones (I am a 17 year old girl). I feel most comfortable looking scruffy, although I can dress quite smartly if I need to (i.e. interview, special occasion). However, dresses, skirts and tops that do not cover my bum are out of the question. Just as are "shapey" clothes. Baggy/army stuff feel best with me, and anything that a neurotypical girl would never wear makes me very happy too.
Firstly, you sound just like me at 17.
Secondly, I think loose fitting clothes can be physically comforting and therapeutic for some autistic people, because of our sensory issues. That would explain why you feel physically sick in certain other garments.
There might be some self-image stuff and identity issues mixed up in there as well (which would hardly be unusual or unhealthy at 17!) but you could try telling your mother that you have sensory needs as an aspie, so you have perfectly valid non-psychological reasons for choosing to dress in a certain way - In fact the therapeutic feeling of loose fabrics against your skin has a soothing effect which lessens your autistic issues and help you face the world better and be a more productive individual.