<human>: Can you give me the gist of the text in a nutshell?

I opened Facebook on my girlfriend's laptop today to see a message from someone I've never heard of/met before mentioning him "fantasizing about us playing again". Her response to the mesage before that (and lack of response on Facebook) make me feel very weird and it seems to me like this conversation heavily implies she is sleeping with this guy. Please check it out and let me know if this seems like paranoia (edit, apparently I can't link to an image in /r/relationships at all. The conversation went:

* Her: "How was your fourth?"
* Him: "It was pretty good, uneventful...it's too bad we didn't hang out so we could 'make some fireworks' :P haha. How about yours?"
* Her: "Haha I know, that would have been fun! One of these days we need to get together :) it was pretty chill, went down to [local park]"
* Him: "Yeah it would have! Yeah we do, we need to soon. I keep fantasizing about us playing again :P" 

I've had suspicions that she's cheated on me in the past and have discovered messages between her and someone else in the past that were borderline, and we almost broke up over those (the text mentioned them getting together and drinking in a hot tub recently, and I had no idea who the guy was) but I let her have the benefit of the doubt and moved past it. Now my heart's racing from the prospect that she's potentially been seeing multiple people without my knowledge or consent while we have been dating.
<bot>: Am I paranoid, or does this conversation strongly suggest my monogamous girlfriend of almost 7 years is sleeping with another guy?