I get severe stomach pain after eating avocados. Is this an allergy?

Possibly. Avocado are in the same family as cinnamon, camphor and bay laurel. You can have an allergy: 1) to the tree-pollen where you develop local symptoms in the mouth and throat shortly after eating avocado; or the latex-fruit syndrome, where you can have generalized urticaria, abdominal pain, and vomiting.