Feeling under the weather can be naturally or purposely (self) inflicted. When it’s naturally inflicted, one can come down with a head cold because of cold temperatures. When it’s self-inflicted, it is because one is feeling intoxicated or has a hangover. To feel under the weather can also mean to feel upset. This idiom is occasionally heard in a professional environment, like when someone is sick and can’t attend work. It is more often heard in social situations.
The little boy couldn’t go out to play because he was feeling a little under the weather.
The manager postponed the presentation as he was feeling under the weather.