>had a relationship with my first gf (her first relationship as well) for 3 month
>we go into one class together (senior HS) and have to go the same way to the bus
>one day she was really happy and even said she knew what she'll get me for christmas and stuff like that
>the next day she's avoiding me and being passive-aggressive towards me
>sounds annoyed when I talk to her (I wasn't being annoying, I talked to her only a few times)
>on our way to the bus I ask her if she's angry at me for some reason
>just says "no" and plugs in earphones and listens to music for the rest of the walk
>same thing next day
>same thing the day after that as well, but don't talk at all this time
>tell her it won't work out between us if she doesn't tell me what's going on
>gives me the standard "We don't fit together."
>break up and ignore each other after that
I didn't ask further, but I don't buy it. One day she was happy and making plans for christmas, the next day she's angry at me for some reason. I don't even know what I did that made her upset.

This was 3 weeks ago and it's still a mystery to me. Do you think I should ask her?