My husband has the same problem, especially when he has eaten something that is "old", like leftovers (stuff that wouldn't make anyone else sick). He has been to several doctors and specialists, and they haven't been able to figure out what it is. He has found that taking a strong ant-acid pill helps alot. He takes one a day wether he is feeling sick or not. You should go to a doctor though, that's not a healthy place to be.
Felling sick when worried is a naturally thing, and sadly I don't believe there is a way to stop the feeling. For being actually sick after eating, I would see your family doctor.
I have the same problems too and I get very nausous after or when Im eating or drinking something. You should talk to your doctor.