Me and my SO have been together for quite some time. We've been through rough patches and worked through them all. We love eachother to bits. Ive told her that she's my first everything (from relationship to sexual stuff) - this is completely true. She told me the same and I believe her.

So the other night we were together getting takeout and I saw her logging in to her account on the delivery app. I saw her password and jokingly tried logging in to her social media account infront of her. The password worked and we both had a laugh because of it working but because we had to do something else I forgot to log off. 
The next morning I woke up and scrolled through said social media site, *thinking I was logged in on my account*, I saw I had a DM from someone and opened it. As I was still fuzzy from waking up and confused because I didn't know the person messaging me I did a quick scroll through the list of contacts to see if I was on the wrong account. This is where any trusting BF may stop and log off but my eyes came to rest on a message where she texted the dude saying stuff like "I love you❤❤❤"

This worried me and upon further inspection I saw that she started talking with the guy during one of our rough patches, telling him that he's the only one for her and even sexual talk.

I'm afraid to talk to her about it as it happened a while back and since then she's stopped talking to her. I just can't stop thinking about it and want to know if I'm in the wrong here