'In a lather' emphasizes a very high level of emotion, more than just being worried or nervous.

Emotional Intensity

The phrase often implies a negative situation where calmness would be preferable.

Negative Connotation

This idiom is informal, so use it mostly in casual conversation or creative writing.

The phrase suggests physical activity intense enough to cause heavy sweating.

Although associated with heat, 'in a lather' can also result from intense stress or exertion.

This phrase can be used for animals, especially horses, after hard work.