Girlfriend Is upset that I want to travel to a carribbean island by myself?

I was planning to go to the Dominican or something for when I finish grad school (been saving for 2y)...
I met my girlfriend 6 months ago and told her about my plan.
I offered to go together but she can't afford it ---shes going to study abroad in Japan and is saving for that...
I said that's okay I can still go alone, and now she's mad at me.
Hardly seems fair... I'm still gunna go, but why might she be upset about this?
Girlfriend Is upset that I want to travel to a carribbean island by myself?
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