Well, when novels are translated poorly, the language might be jumbled, the meaning gets distorted, and you have to constantly struggle to make sense of it. That's what gives you a headache.
It could mean that the person's thoughts or mental state is like an animal story, full of wild, instinctive or untamed ideas. Maybe it's a creative way of expressing that their mind is filled with stories related to animals, like adventures of animals in the wild.
There could be a variety of reasons. Maybe they miss you and want to know what you're up to. Or it could be a subconscious habit. It's hard to say for sure without knowing more about your specific situation.