What does it mean if someone says 'I'm under the weather'?

Their mood is easily affected by the weather.

What does it mean if we say we have 'fallen head over heels' for someone'?

It means we have fallen deeply in love with this person.

It means this person has disappointed us in a big way.

What does it mean if we say 'I'm over the moon'?

It means we have surpassed a situation and are no longer sad about it.

It means we are extremely happy / ecstatic.

It means we have just made a lot of money.

What does it mean if we say something happens 'once in a blue moon'?

What does it mean if we say 'we caught someone red-handed'?

The person was seen doing something they shouldn't be doing.

The person was falling in love and we noticed it.