Is it possible for a healthy 3 year old Siamese cat to have an
allergic reaction to shellfish (canned whole oysters and canned baby
clams)?

My Siamese cat this January after vomiting and not being able to keep
any food down for a couple of days, was diagnosed with an inflamed
bowel.

Last week after another vomiting session, x-rays, barium, exploratory
surgery found the small intestine "hard as a garden hose," biopsy
samples.  Today I am still waiting for the laboratory test resusts. 
(I hope it is NOT cancer.)

Aside from the popular dry and canned food, we fed the cat alot of
shellfish.  He particularly loved oysters!

I wouldn't assume that a shellfish allergy is responsible. Certain
kinds of intestinal problems are more common in Siamese than in other
breeds. It just isn't possible to do very accurate guesswork over the
Internet.

I hope the lab results come quickly. I certainly know how distressing
it can be not to know what is causing the illness of a beloved pet.