I think you should tell her the truth about having to work (you are telling the truth riiiiight?

) but also let her know that you are still wanting to interact with her.
I say you meet her for lunch the day of the show or maybe the next day. Besides the whole idea is to spend more time alone with her (which can be attainable in a concert/show setting but not as likely considering there will be music blaring and not many places to be alone (depending on the venue)).
Also, I would recommend you call her instead of SMS messaging her because your vocal tone can convey interest and it won't seem like you are hiding behind words/avoiding confrontation. Just be honest about your work situation and have a rain check date already planned so you are still leading the interaction.
If you checked off all of Attraction Phase (which I assume you did considering you got her # and she invited you to that show) then I don't think it will be an issue.
Good Luck!