It can be really difficult to feel like you don't care about what's going on around you. I've experienced this too, both during high school and now at university. Finding any motivation to keep studying was a struggle and I was no longer interested in trying to connect with people. These were both things I'd tried my best at previously.
It seems like seeing your family and grades were things you cared about in the past, or at least felt something towards. This is probably one sign that you're not making something out of nothing. I'm constantly worried about this myself and it can make it difficult to see someone about how I'm feeling.
Not feeling anything or caring about things you used to care about can definitely be a part of depression. It can be worthwhile sharing this with a psychologist, especially if it's helped in the past. It's unlikely you're over thinking this since it sounds like you don't feel like yourself.