Why would my girlfriend not want to see me on her birthday?

is it really a big deal if she wants other plans for her birthday? she wants to go out of town and visit friends and I don't think id be invited.

ill be able to see her the day before she goes, so maybe its ok?

should I be pissed about this? should I be OK with it? is it a big deal or not?
Why would my girlfriend not want to see me on her birthday?
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