Tiredness, hangover, cold weather, perhaps a cold coming on, working too hard, general "over doing it" can be many factors. You mentioned anxiety, do you suffer with anxiety issues? I had a bad year last year, work and relationships were just too stressful and I got a bit low. I found that my anxiety was increased when I was feeling tired or hungover. Dont underestimate how much the body needs rest, the world seems a lot harder when you are tired and it can even bring on physical symptoms.
Let us know how your day goes!!