I feel like an old man

I don't know why I feel so weak today. I slept well last night and slept through. The morning was good, my headache was almost gone. Now in the afternoon I'm all trembling and a bit dizzy. Feel like an old man. I wanted to last whole day. I hope I can make it until 20h... Then need to hop in my bed again and rest my weary body. So annoying O_o