Why do I feel like this?

So my SO of a year and a half just told me he thinks him and his friend are going to move in with his friends brother in fall, we’re both 20, are in university, and both currently living with our parents. And I’m not sure WHY but when he told me this, my heart kind of sank, and I have no idea why. He would be living like two blocks away from where i live now which is great! And I feel like his parents are very strict so I think it would be super nice for the both of us, but I still don’t know why my heart sank? Maybe just because i thought him and I would move in together first? I’m not sure, if you have any thoughts, please share!!!