(translated this from my language to English with ChatGPT, using a throwaway account aswell. TLDR below)

I 30M was starting to feel like something's off with my girlfriend, Amy (31F). I've discovered that she's been flirting pretty intensely with a guy from the gym, Mike (32M). She suddenly started taking her phone everywhere with her, whereas before she used to leave it lying around. She got nervous when I used her phone (for example, when mine was dead) and would look over my shoulder the whole time. That raised a red flag, so I asked her about it, and she said she just values her privacy. Fair enough.

But I started noticing more things that made me feel like she was hiding something. Like how she went for a walk with this guy from the gym without telling me. She started frequently asking if something was wrong when I couldn't sleep, being extra sweet or “in love,” and spending more time at the gym than usual. These are all behaviors I've seen from her before — twice, in fact — when she was flirting and hanging out with other guys.

So I ended up checking her phone one night while she was asleep.

Turns out, she and Mike ride their bikes home together from the gym, and before splitting up they secretly cuddle at some spot they now refer to as “the cuddle place.” I feel ridiculous even typing that, honestly.

They send each other messages and selfies like they're dating — nothing nude or explicit — but still. When Amy went away for the weekend with her mom, she sent him a picture of the bed saying there was still room for him, and asked if he wanted to come over (jokingly, but still). She even bought a souvenir for Mike, and not for me.

They talk about their erotic dreams, which apparently feature both of them, and they’re both looking forward to an upcoming gym trip to Denmark. Mike joked that Amy would fit under his sleeping bag, and she recently bought a two-person hiking tent. They’re curious how flirty they’ll be once there’s alcohol involved. I could go on and on with more of these flirty examples.

So far, it's technically innocent — but I wouldn’t be surprised if it eventually leads to them ending up in bed together.

1. Am I overreacting if I break up with her over this? This has happened before — twice — but both times were shorter and less intense because I found out early. This has been going on for six months now, and it’s like they’re counting down the days until Sweden. What would you do?
2. Should I tell Mike’s girlfriend? I have her contact info.

TL;DR:
My girlfriend has been secretly flirting with a guy from the gym for six months — cuddling after workouts, sending flirty texts and selfies, talking about dreams they have about each other, and planning a cozy trip to Sweden together. I found all this on her phone while she was sleeping. Nothing physical yet, but it feels like it’s heading that way. This has happened (in milder form) twice before. Am I overreacting if I break up with her? And should I tell the guy’s girlfriend? I have her contact info.