AIO for asking why money was taken from our account

Alright so for context I, F21, share a bank account with my GF/Fianceé, F22, she has her own separate account as well and I'm going to set up my own separate one but as of right now she has access to it. I just got paid today but when I went to check I was a $100 short and so I at first thought that it was me not plbeing paid the right amount but then when I went into the history I saw that it had been removed. Now when my GF gets gas money she uses her own money not one from my account/ the joint account. Which is why I'm thinking something else is up. Am I overreacting or is this actually something?