I think pecans make me feel really sick?

Being a long time food allergy sufferer, I can tell you it sounds like you are definitely allergic to the pecans to start with. Wanting to vomit is a symptom of this. If you really want to be certain, go to an allergist to get your blood tested for nut allergies (incl. the specific one you are experiencing). Better to be safe than sorry, it is more dangerous to go around not knowing. Good luck!