Hormones. Lots of them. Coursing through your bloodstream at the speed of sound.
Of course, the medication-insane world will have you believe that there is something wrong and prescribe a high-priced pill whose side-effects are worse than the so-called problem. But, really, it's just hormones.
If it's really a problem, you can get into relaxation exercises. They are good for keeping a mental balance.
Agatha Christie is part of an interesting theory. Apparently, she was not a great writer initially. And then she disappeared when she divorced her first husband. And when she reappeared 11 days later, she was a great writer. I have read analysis of her first novel compared to her later ones and they say that they weren't written by the same person.
Robert Lewis Stevenson is another author with a similar story. He disappeared a poor writer and reappeared as a genius.