“What causes diarrhea immediately after eating?”

Gastro colic reflex

Malabsorption- lactose intolerance, rapid GI motility, food content

It’s a very long discussion and may be best if discussed with your treating doctor.

We all have a reflex called gastrocolic reflex, which prompts the rectum to empty when food arrives in the stomach. If you are otherwise healthy, what you are experiencing may be within the spectrum of "normal." It may also be a symptom of Irritable bowel syndrome, which is very common. Some more serious conditions such as inflammatory bowel disease need to be considered. If it just started within the last few weeks, a bowel infection is a distinct possibility. Please consult your PCP regarding simple stool tests to eliminate more serious conditions.

In an otherwise healthy person, this is caused by a gastro-colic reflex. It's a neurological reflex mediated by the vagus nerve. When the stomach fills with food, a reflex signal is sent to the colon causing a contraction, hence the need to have a bowel movement. That contraction will force out stool even if it is liquid. This is the body's way of clearing out the colon in anticipation of the meal eventually getting to the colon.