M.C.
It sounds like a hormonal thing to me (given that you are not "usually" like this, and you aren't going through anything particular stressful right now). It could be a pregnancy... you might want to take a test if there is a possibility- even remotely. It could just be your body adjusting...
I have depression that is aggravated by seasonal changes... so in the winter I have to make an effort to get out and get some sunlight. If you have been indoors a lot lately, maybe you could try spending a little while outside in the sun... and see if that helps.
However, if this persists for more than a week or so- go get checked by the doctor. There are all sorts of things that could be going on in your body to affect your mood. It could be a vitamin deficiency, hormonal thing, diet issue or something else. A lot of times they can check your bloodwork to find out.
Good Luck!
-M.