My Mom and I went shopping.
First she looked for clothes for herself.
And then we went to look for clothing for me.
However, everything was either too big (my Mom's size) or too small.
So my Mom would pull out the things that were too small and tell me I should be able to fit in it.
But I would tell her I was obviously too big for those sizes.
However, they were the sizes I used to wear about a year ago (When I had an ED).
And she was making me so upset, because she kept insisting I could wear them.

Then I woke up.

So, I think this dream might be that I think I'm fat.
I'm not entirely sure though, so some input would be great.

Never go shopping with your mother in real life. If you think you had a complex before...

As the scene is focused upon clothing, the main issue here is that of your way of functioning and behaving.

The scene suggests that when in a situation in which you can buy or accept any particular way of functioning or behaving (shopping for clothes), your personal belief system (Mom) appears to have priority. (First she looked for clothes for herself)

Clothing that is either to big or too small would in general refer to a situation, function or behavior that does not fit, or is not suitable, for you. Using your personal beliefs you are choosing for yourself functions or behaviour that is definitely not suitable (Mom would pull out the things that were too small) The too small clothes can imply a function or way of behaving that has been outgrown. This seems particularly emphasized by the clothes being the size you used to wear a year ago.

[I am assuming that “ED” refers to an eating disorder.] By relating the too small clothes with a time when you had an ED, the function or behaviour that you need to see as being outgrown would relate to the behaviour engaged in at that time. The scene indicates that the True or Inner Self would be very upset if your personal beliefs were to insist on using this way of behaving here and now because its time has past.

The too big clothes, on the other hand, show that you could choose a new way of functioning which may seem not perfectly suitable now but which you can grow into later and be very comfortable with it.

All dreams are meant to point out some error in one’s thinking. Your dream is pointing out an error in a certain type of behaviour (the ED thing) and also showing that you may have a serious misconception as to the actual clothing size that you think you should fit into – a misconception caused by your own beliefs.