why am I still tired

I feel like I have no energy at all. A couple of times I felt like I was going to fall asleep at my desk at work. I don't know why I'm so tired. I've been sleeping well. And on the nights I can't sleep (which only seems to happen on the weekends), I actually don't feel exhausted the next day. I've had this problem for a while. It's just got worst last week and this week. I've also been feeling like I'm going to throw up lately. That only started last week.