You definitely are not alone - I have been feeling like this on and off for a month now. And I used to be the jolly one in the office.
Now I'm just the quiet one sitting in the corner, blocking out the world with her iPod.
I just have to deal with it, knowing that I have to keep my job and that this is just an episode and will pass. And my mood-stabilisers, which I've just started, will kick in eventually.
And I have 2 more subjects where I write exams end of May to finish my degree - I have not yet opened a book because by the time I get home I am so emotionally drained I just chill in front of the TV and go to bed as soon as possible. Just know that this feeling will pass and be strong while it lasts