That seems more like depression, to be quite honest. The inability to explain it might be alexithymia, but not being unable to care or get excited for things sounds like depression.

Try imagining this.

You just got some great news. Life-changing news. When telling others about it you don't act or speak excitedly because you only have a dim feeling of happiness about it. However when climbing the stairs in your home you effortlessly bounce up them and that night when lying in bed trying to sleep your thoughts patterns are all short & jumpy and keep returning to the good news.

You're having many symptoms of happiness and excitement so the feelings are happening on a biochemical level but they're just beneath conscious awareness. It's the physical symptoms without feelings which is the tell.

Of course most of the time it's much harder to notice physical symptoms because most events are not that big of a deal.