Hey, I'm 17 and kinda new to that TRP stuff but I learned a lot from it already, thank you everybody

I was at a music festival with my girlfriend (and friends). There was big dusty area with sprinkler to allow people to "bath" in mud and have fun I presume. I'm not a fan of those things, my gf is though. My problem is that she went near the sprinkler few times with her friend, while I & other guy stood back. First time, they went with shirts on, they came back with them rolled up. Second, they arrived with shirts off, only bras. I didn't like her showing that much of herself (she said earlier that "I'm too shy to walk with only bra on, blah blah"), told her that in pretty beta way I guess, got shut up with the "you really believe I could take off my bra, you silly bf?" question.

I know I'm probably overreacting and overanalyzing this event, but how should I let her know that I don't like her walking around with only bra on? (She's 16, I think that matters in this case)

Or I want too much control over her?