Mushroom trips often exaggerate a person's focus, so by maintaining attention on the sensation makes it worse. In an overheated house where I was sweating, I was convinced I was cold because I was focusing on the winter snow outside. But when I went outside, I wasn't cold anymore. Don't think about shitting.

Physiologically, the body only responds to large doses of a serotonergic substance initially due to the sheer volume of serotonin receptors residing in the gut, it's to protect against consuming poisons where the correct reaction should be to vomit. Once the substance is absorbed and the consumer intoxicated, and nausea is typically reduced.