Why's my GF being so hostile?

I'm telling you, there's nothing I can think of from the last 3 months that would suggest she has something against me :/

I hope it's just the stress talking indeed ... I don't know how I could even mention to her that she should go see someone about it without sound patronising and make her mad ...

Your GF says something harsh and you make a thread about it? Have you thought that you yourself are overreacting? Everyone has days when they are stressed and when they make mistakes - we're humans not robots. If you keep analysing every word she says, you're doing no one in the relationship a favour.

Why don't you just forget about it and move on? You keep saying that you can't think of anything you've done but if you keep talking about it that way, it sounds kind of clingy (sorry) as in, looking for constant reassurance and insecurities.

(Also, you say that the relation is 'fantastic' and then you say that your GF puts hardly no work into the relationship? Is this just being defensive or something deeper? If there are issues you are unhappy with, this may affect how you hear what your GF says plus leaving issues unresolved just makes them boil over and much worse to deal with)

She has recently failed an exam, with the next being tomorrow, and she is telling you that she hates people that don't take education seriously. It don't think that it could be much clearer that her attitude is coming from mounting exam pressure. Your complaints about what may seem to her right now to be a relatively small issue have simply rubbed her up the wrong way.

Seeing as you say that everything else has been great with her recently, I would not read anything more into it than that. I think that all you can do now is try to be supportive during in the aftermath of the exam tomorrow.