In my opinion
I'm not sure how worried you should be if it's not the first time, it's already happened so that's enough. The future is more concerning. Would you ever be able to trust while she goes out of town again? If she went back home or to a sporting competition would you be worried shed do this again? I know for me personally I would be too anxious, trust is really important.
The fact that is what matters to her right now is alarming. If it's something she wants you to look past and forgive her for why does it need to be such a secret for her? The only reason is because she's embarrassed but she doesn't care that she just made you look like the dumb one.
This would be significant enough for me to end a relationship. Trust is really important..