Why do I feel so tired and weak? I've slept well and eaten well.. what's wrong with me?

[lazydazy]

I feel like I could sleep again, and I don't understand why I didn't even oversleep. Also I feel drained, tired and weak. I have felt like this for a while and this feeling hit me a bit later in the day, then it hit a few hours after I woke up (although I was still feeling sleepy etc.) and now it hit right after I woke up. What is going on?