My girlfriend (22f) wants to break up cause I said nothing when a guy hit on her

So here's what happened: We were at a party and my girlfriend went outside without me for a few minutes. I was still inside hanging out with friends. When she came back, a guy followed her in. He came up and talked to her for a few minutes, asked for her number and she gave it to him. I still had no idea what was going on. Then she comes up to me and starts crying. She tells me that she was outside and the guy came up and talked to her, flirted with her and she told him she was taken. Then he said something along the lines of "I'm sure you'll give me your number if I hit on you enough". I guess she didn't like that and told me about it, asking me why I didn't say anything.<br><br>That brings us to now. She's been absolutely furious with me since then and saying that I should have noticed and protected her. I don't get it though, she can handle herself, I was inside talking to friends.<br><br>Edit: I'm 100% sure she was out there for more than 5 minutes because I was having a drink with another couple of friends and she's was gone before I had the drink and came back after I had finished drinking it and the guy came with her. Also, she's been yelling about how I can never see when she's upset unless she mentions it. Also, the guy was going for her number and she was giving it to him, she didn't look upset. That's all that I noticed.