First and foremost, she's not in the wrong for how she felt in the moment. Odds are, she probably did want to go to the concert but when she got there she felt immediately uncomfortable.
Secondly, lets not throw away her feelings because she probably had a fight or flight moment and because she had bad moments during a mosh pit before she instantly wanted out. That isn't your fault, but it just happened to kick in and she ran out of discomfort and probably fear.
Thirdly, I don't want throw your feelings away or not justify your feelings. You have the right to kind of be like "but what the fuck" however, I think you need to understand she couldn't communicate her feelings because she probably couldn't vocalize them. Her body and emotions communicated her thoughts or feelings in that situation and you need to be understanding that in high tension moments like that, you just need to understand and learn how to deal with that in the right way.
Finally, what you should have done is at least have a secondary conversation before the show's date came around to ensure she's comfortable and doesn't have any second thoughts. That way, you show that you're understanding from her past situations and that you don't want her to feel any discomfort at all. Also, you're not an asshole for not being able to find her. It's heavily populated situation and you're not a magician who can suddenly find their girlfriend who happens to be small in a place probably filled with 100+ people if I had to guess.
(edit) Of course this is my opinion and I could be highly wrong, I don't want to seem like I'm throwing either person's emotions away or trying to not justify them.
^ Pretty much my feelings.
Think of how she felt. She sounds like she was uncomfortable with all the bumping and the amount of people around you. She obviously didn't feel comfortable even though she thought she would when she agreed to go. That's something that can happen.
The screaming at you was also an emotional response as she sounds like she was still scared/shaking/etc. People can say it's immature all they want but everyone, adult or child can and do have outbursts like that even if they don't want to admit it.
If there was that many people around, I think she may be a bit claustrophobic maybe? In other words, it's best to avoid situations like that where you'd have lots of rowdy people all around you so much that you'd have trouble moving without bumping into people like this.
My personal suggestion would be to talk to her, apologize for not considering her feelings more but if you want, you can also say "I didn't appreciate being yelled at like that in public. I understand why you did it and that you were upset and I should have thought more about you/looked for you/texted to see if you were already but it did upset me as well. that you would yell like that at me in public"
something like that. It means you get your feelings out about how you felt but also shows that you have an understanding of what she went through and that you'll try not to let something like that happen again.
Something like that anyway. I just... I can honestly see myself in both peoples shoes, yours and hers as I doubt I could handle that situation either and I am a pretty emotional person but I'd also be angry about being yelled at like that in public.
So yeah I feel for both of you.