(I've) been under the weather
(I've) been mildly ill. Yeah, I've been under the weather, but I'm feeling much better now.
(I've) been under the weather.
Fig. a greeting response indicating that one has been ill. John: How have you been? Sally: I've been under the weather, but I'm better. Doctor: How are you? Mary: I've been under the weather. Doctor: Maybe we can fix that. What seems to be the trouble?