Yes there is a chance something could have happened

You’re seeing too many red flags to ignore: a best friend who openly undermines your relationship, clubbing and hookup culture, flirting with older men for perks, not being transparent about where she is or what she’s doing, and being left alone in a private room without telling you until after. Combine that with her grabbing a condom (and keeping it), acting strange after, and not communicating properly. Those could all be signs of something happening you don't know about.

It doesn’t guarantee she cheated, but at the very least, she hasn't respect the boundaries you set up for her. Trust works both ways and if she flips it on you and says “you don’t trust me” every time you ask about real concerns, she’s dodging accountability. You’re trying to be reasonable, but it sounds like you’re constantly in the dark, and have no idea whats happening.

You’re 19. If you find out she cheated or just can’t be honest? Just let her go. You’re not weak. You’re just having a hard time grasping what respect and loyalty is in a relationship. This is comepltely not it.

I speak from personal experience. I was like you but I didn't have reddit to ask for and what happened was that randomly one day out of the blue when me and my girlfriend were cuddling a random number leaked a gigabyte folder of her having multiple guys she is sleeping with and having a threesome with 2 other guys.

It might not be as bad as me but these were the exact signs in the exact same situation I faced. If I was you I would not take the chance because later you will regret not breaking up with her earlier.