I personally infer that the nickname popcorn has the following possible reasons or meanings:
1. I particularly like to eat popcorn.
2. Have watched and liked popcorn-related movies, TV series or cartoons.
3. There are words in the name that are homophonic or the same as popcorn, or words with the same meaning.
Four. I personally had an afro hairstyle, and it looked like popcorn.
5. She has special experiences related to popcorn, and popcorn has a special meaning to her.
Basically these are the possibilities. It is most straightforward to ask your girlfriend directly what it means.
Just ask her directly: Is your nickname Popcorn because you like to eat popcorn? I'll buy you whatever flavor you like, and I'll buy you as much as you want.
Normally, if you ask this, she will naturally tell you the reason and meaning of giving herself this nickname.