体調が悪い

in a bad condition : 身体がだるい

I am in a bad condition, so I will take a day off.

take a day off 休みをとる

Under the weather : 体調不良の

I have been under the weather for a few days.

be/feel under the weather

Not feeling well : 具合がよくない

I'm not feeling well, so I will stay home all day.