Most Common Causes. Some of the most common causes of feeling sick after eating stem from the content of the food, though some disorders can cause constant digestive discomfort as well. 1  Food poisoning-If the food you have consumed was contaminated with bacteria or fungus it can lead to nausea. Noting what you have eaten when the nausea occurs can help you narrow down this condition. 2  Improper eating-Eating too much too quickly or failing to chew your food thoroughly can cause you to become nauseated after you eat. 3  This may also cause indigestion or heartburn.