I'm not entirely sure as 'fock' might be a misspelling too. If 'fock' was meant to be 'fuck', it would be inappropriate language. But if we assume it's just some made - up or misspoken phrase, it could potentially mean a girl is involved in a big, perhaps complex or elaborate story.
Well, without more context, it's really hard to say precisely. 'Doing big' could imply being engaged in something significant. However, 'fock' being an unclear or misspelled term makes it difficult to fully understand. Maybe it's a very local or personal expression that we're not familiar with.
If we break it down, 'girl' is clear. 'Doing' indicates an action. But 'big fock story' is quite ambiguous. It could be that the girl is in the middle of a large - scale or important story. Maybe it's a story she is creating, or a situation she is part of that has a lot of elements to it. But again, the 'fock' part is a bit of a mystery and might be a mistake.