I feel slightly weird for posting this because I'm not actually a parent, but I read quite a few of the threads on here and it seems like a really good place for long-term relationship advice. 
I have been with my partner for a number of years now and I think things are going well (no red flags). However he has been socialising in his home city (where he also lives) without me for a while now. Nothing to be angry about but this is very different to earlier in our relationship when we often did this together. Also, I am living nearby in a small town so it's not like there are obstacles. As stupid as it sounds I only realised all this today when he told me by text that he was going to his friend's gig tonight. He stayed at my house last night and knew I had no plans this evening. It suddenly hit me that I hadn't seen any of his friends or been to a social event like that with him for ages. This probably sounds dumb but the change has been gradual and we've both had to deal with moving for uni and jobs in the recent past, plus we do a lot of daytime/night-in things together so I didn't notice this part of my life had trickled down to nothing.
I think it has upset me because I feel I have been hidden away over time or compartmentalised, and I'm sad that I didn't even notice it was happening. We used to have a good time hanging out with his friends and going to gigs and it felt like he was proud to be with me, so now I am wondering.
Also, we have talked about living together (which he suggested and is enthusiastic about) and it would be in a different city because we are starting new jobs. But now I am worried that I am walking into something where I will be sat at home while he's out socialising without me. BTW I do also have friends and go out but I often include him in what I do.
I did text him after he told me his evening plans and I said I felt sad that we didn't do that kind of thing any more... etc and he said he was sorry and glad I had told him and he would invite me to things and he is proud of me. He seemed to be very compassionate rather than defensive but even now I'm wondering about that. Which way would a dishonest person more likely react? At the moment I don't think there is anything bad going on and more likely it's caused by one or both of us letting things slip. Has anyone here had a similar experience? Do you think this is just natural relationship change? Any advice? 