Am I in the wrong?

Currently back for uni for another week or so, been back since last Wednesday. Obviously I've been seeing friends from back home who aren't at my university, which includes making plans to meet some next a weeknight evening. Girlfriend (whom I spent all weekend with, as well as living with at university, rang me the other evening to ask if I want to meet her and her friends next week (on the same evening as I've planned to see friends who I don't see when I'm at university). I told her I'd like to come and meet her friends but as I've already made plans I'm not sure if I can, I'd see what I could do. Anyway, I told her this morning that I won't be able to make it and since this morning she's been extremely blunt as apparently I'd claimed I'd 100% go (absolute rubbish). Am I right in thinking she's being extremely petty, especially as I see her daily when at university. More of a vent more than anything as I can't really see an argument for her stance (it's not even blob week). anon, think she's on here.

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