I went through the same thing a good while ago, Tilly. I know there may have been, in the past, reasons for mine but it was unusual for me not to cope and to be having these feelings.
I think I let it go too far but when it turned into a strange flatness that made me feel as if not only could I not be happy and react to good things I also felt that if the worst thing in my life were to happen I wouldn't care.
That made me see the GP and he was great....luckily he knew me well and knew this was way off for me. He explained about the chemicals in my body and why they were changing and affecting me in this way.
I tried some kind of cognitive therapy but believe it or not I am not good at talking and opening up....really....and it did no good so I had a course of Citalopram. That worked really quickly and made a huge difference.
So it could be a mild depression and, as I came to realise, it's very different to being down because something in your life is wrong....
Do chat to a good GP because the earlier you start to fix it the better.
Gx