One way you can tell if it's the winter blues is get a Vitamin D3 supplement. Take 2000mg daily (the bottle will say 1000mg, but that is the threshold for feeling the effect, not the least effective dose, kwim?)
Give it 2-3 days...see if you notice a change in your mood. I'll bet you do. :D
Of course there are SOOOOO many other factors like diet and exercise, "life stress" and major changes. Don't sit on this for too long. If you've had a long cold winter w/o sunlight though...D3 may be where it's at!