Should I consider it a cheating?

Intro: I’m in a relationship for 3+ years with my gf. We really love each other, being open, talk a lot and I’m feeling that I found the very right person for me. From her side I’m feeling the same thing.

Problem: I found some questionable (at least) chat of my gf with a guy who she met during her trip with her friends. I know about him, she told me how they met each other and other details, so I was totally ok with it.

Before telling what exactly I found I’d like to mention that I feel being guilty of even seeing this chat, but I’ve got her phone with her consent (we both normally do it) and was in the chat app as she asked me to write some message to her friend (other one). And I just saw some suspicious last message in the chat and I opened it later and I saw the ending of the suspicious talks, so I scrolled for a little and found the following (then in a few hours I’ve returned to the chat to preserve the data so I’m not mixing some emotions and words).

… Gf: there’s a storm in our country Guy: oh, it will be soon in our country as well Gf: I got caught in this storm yesterday in a white dress Guy: Say hi to your friend. like the dress got wet and everything was visible? Gf: yes. it was too short. didn't get along with the wind Guy: haha. and tell me that you were without a bra and everything was visible. I got home on time today… Gf: it is not necessary to know such details ☺️ Guy: Haha. ok, keep your little secrets Gf: you almost guessed the brand i was wearing Guy: Haha, [GF’s name] Secrets? Gf: the secret is how they sell such crap for such money Guy: what, it's not pretty? Gf: Beauty is subjective, but quality... Guy: is it bad? Gf: [1 min of voice recording where she tells how overpriced it is like underwear , bras etc] (p.s. all of it was my gifts to her from our trips) Guy: omg, for such price it would be better to travel somewhere and to go without panties Gf: only when man stop from existing[sarcastic]

Besides the too intimate discussion I noticed a few more disturbing things: she has no white dress at all, that day she went in trousers and t-shirt (not a stalker, I just had her photos from that day as we’re taking the photos of each other’s looks so it is easier to know what to be dressed with later). And that day she was less likely in her VS underwear as she wears it on special occasions mostly. So it makes the discussion even worse for me, because I can’t pretend she was unintentionally getting into details that were real, she was just making it so. I can’t imagine such communication with any other girl besides my gf. Please tell me if you see any reason for this kind of talks being normal in non-open relationships.

A few days later she was drinking in the bar with friends and she was texting with him.

Guy: [GF’s name] secrets (seems like reminded as a joke Gf: Why? Stupid autocorrection. Guy: haha, your fingers don't work, they don't obey mommy? Gf: it's horrible how vulgar it is Guy: haha, yes, I agree Gf: [changes the theme]

So that is the picture, rest of the talks seems to be normal. I’m too suspicious and unstable right now, but it seems in a few places messages were deleted, but I’m not sure, maybe it’s just my current state.

If she’s flirting with him, then why do they have only these two intimate discussions. If they’re flirting then why she stopped him when he responded. If it is not flirting then why she continues the communication at all, I’d just stop talking to such person.

I’m asking for your opinion for this situation. I just feel like I want to see something positive here. Maybe I am too protective, maybe I was not giving her something. Please let me know what you would do in such case. Should I talk with her? Should I simply ignore this things?

Thanks in advance