under the weather: If you are under the weather you don't feel very well. You could have a cold or a hang over - nothing too serious. "Under par" is another way to say this. If you are sick, in the UK we also say you are "paulie"

A:"Is Julie coming in today?"
B:"I don't think so. She looked a bit under the weather yesterday. I think she's caught that bug that's going around."
A:"Well, perhaps we should give her a call, see if she's OK. And in the meantime, can you type this out for me."
B:"Of course. You look a bit paulie yourself, perhaps you shouldn't be working either."
A:"Oh, I'll survive."