What might be causing these symptoms?

For the past few days I’ve been very moody—depressed and irritable. I’ve had increasingly severe fatigue, and now, nausea.

I had a really hard time getting through my normal workout this weekend, although I did manage to push through. I have a very early Monday, so I didn’t get much sleep (5 hours), but that’s the case every week, and usually I function okay. Today, I had to leave work, nap, and was still useless for the rest of the day. I have deadlines and can’t afford to be out of commission tomorrow, too. What might be going on?