<human>: Give me the gist of the text in a nutshell

Just looking for some general advice on this.. We all have X's, some are friendly and some aren't, and surely we've all said a brief hello to them every now and again. Casual or what not. 

However, my girlfriend of 1-year found an X on FB she said (I think, or he messaged her) and they talked back and forth from 4-11pm. Like a shit ton of messages. then she messaged him again this afternoon & have been off/on for the past 4 hours. A lot of it was regarding a similar hobby they have, but still. He lives halfway across the country, so I'm not worried. But that's a lot of talking.. A lot. She mentioned she was talking with an old friend, but doesn't know that I know it is an X. 

He hit on her multiple times, even though he's married with 2 kids.. She did however, bring me up. As her "amazing bf" once or twice, & made sure to mention how happy she is & that I'm a wonderful man. He said he was happy for her, but very jealous cuz he's unhappy in his marriage . She said "well you made your decision.." Then they continued talking for another few hours. 

No, I don't ever go through her phone as I feel that's not right. But this was in FB messenger on our shared tablet.. I guess there's nothing wrong with talking to an old friend, but that's a lot of talking. & she kinda hid it. I'm more concerned with the fact that she hit it, & the amount of conversation.

I'm extremely comfortable in our relationship, & trust her, but it's been on my mind all night. Do I bring this up at all, or ignore it as friendly catching up? 2 days is no biggie. If it continues long term, I won't like it.. 
Thoughts input & advice would be much appreciated.
<bot>: My girlfriend has spent the last two days messaging with an ex for hours on end & hid it from me. Do I brush this off, or bring it up if it continues? He lives 5 states away.