['My girlfriend has been messaging another guy from work a lot recently.', 'do you think they are up to something?', "I can't help but feel like they are.  I have asked her about it, and she says they're just talking about work.  Maybe I'm just envious of the attention she's giving."]
You should be careful about jumping to conclusions.
My girlfriend has been messaging another guy from work a lot.  When I ask about it, she says they're just talking about work stuff, but I can't help but feel a bit weird about it.