When I eat shellfish, it makes me sick. Not the typical hives, swelling, itchy etc sick. Almost immediately, I feel like I need to have a bowel movement but can't. Then I vomit. I will feel a bit sick for about 10-20 minutes and then I'm back to normal. I didn't always have this problem. I noticed it about 8 years ago. After having the same thing happen after eating shrimp and then crab I concluded that it was the shellfish. Is this an allergy?