I (17m) don't know what to say to my girlfriend (16f)

Okay so little bit of context, me and my girlfriend (16f) are in a long distance relationship and she is currently on a trip to Florida with her best friends family, and it was a 19 hour drive so they stayed in multiple hotels on the way there. She is also my first girlfriend. I got her to quit smoking many months ago, as I absolutely despise it. She did smoke one other time while her old friend that moved away came over, and I forgave her for it and just moved on. That was a while ago, and I thought she'd never smoke again.

Today we're texting and having a completely normal conversation and she randomly just says that she's outside with her friend and her friends brothers girlfriend, and everyone is smoking except my girlfriend. She says "probably not". I was kind of confused why she only said probably so I said "oh" and she asked why I said oh, so I asked "only probably?" and she ends up telling me that she smoked the past two nights. I kind of paused just wondering if she was joking or something, but I guess she wasn't. She told me she's sorry and that she was gonna tell everyone that she wasn't gonna smoke tonight. I don't even know how I felt, I wasn't really mad, just really hurt. She said "I'm sorry please don't be mad at me", and I just said it's okay. At that point, I didn't even know what to say to her. I haven't been in the best mental state in a while, and we haven't called in 4 weeks. That might not sound like a while to you but we usually call every night. I was just so lost and I couldn't reply. She said that it was really hard not to smoke when everyone else is doing it, and I didn't really understand because wouldn't she just be used to not smoking anyway? I asked her that and she said that's not how it works with smoking and that she thinks of it all the time. When I asked why she didn't tell me the first night, she said she thought I'd be mad at her. I told her I'd be less mad if she told me the first day and she just kept saying sorry. Eventually she said "i'll try not to do it anymore and i promise i'll tell you if anything happens" "i know i shouldn't have waited to tell you im sorry" and I just said it's okay. I couldn't think of what to even say. After a bit, she said she's gonna talk to me a lot more, but I told her that she should enjoy her little vacation, and then just when I'm feeling better, she says something that ruins my mood again. "I can't enjoy anymore" "before i told you that i smoked i convinced myself that it would be okay to smoke for this whole trip and now that i've smoked twice its gonna be really hard not to" I just stared at this message for a few minutes. She promised me months ago she wouldn't smoke anymore and now this?

At the end she said she's sorry again and that she'll change the topic because she could see I was getting upset and we just talked about whatever until I felt better, she stayed up until she could make sure I was okay, and then went to bed. I can't sleep though. Its 2am for me right now and I'm genuinely freaking out about all this, and I don't know what to do. Am I overreacting?