My girlfriend is moving into her first apartment this week with 4 other flatmates. They're all in their early 20's and pretty much away from home for the first time. 2 guys, 2 girls. I met all of them already except for the one guy.

The one I haven't met yet is from out of town. She knows him through an event her and her family have been attending for the past couple of years. They've been hanging out among the same circle there, but haven't had contact until recently, where they both figured out they were moving into the same city and since she was looking to fill the ranks for the apartment, she asked him if he wanted to move in with them.

He came up last week to put his signature under the lease and they spent that whole day together and he spent the night at her moms house while she stayed at her dad's place. So far so good. Yesterday she came over to my place couple of minutes late, while talking on the phone. I could just make out a man's voice so I asked her as casually as possible "was that your dad?" Nope, it was the guy from the flat. She started going on about how and what. It didn't matter to me much, I just found it unbelievably disrespectful being late and talking on the phone.

I didn't say anything about it to avoid her thinking it was because of the dude, but the empty, feinty feeling of jealousy started creeping up on me. I had the opportunity to step out for a few minutes to regain my composure while I was out to buy new condoms and fucked her afterwards. In moments like that I dominate her as much as possible.

As I said, I don't know the guy yet and I'm not gonna start fighting against ghosts. I'll meet him soon enough and I most definitely will not plant a seed in her head that I'm insecure and that their might be something developing between the two (which I suspect their is, she just doesn't know yet). It definitely feels off and I need some advise how to proceed from here.

TL;DR My girl is spending suspiciously much time with her new flatmate, need advice on how to handle situation.

EDIT: formatting / spelling