Whenever I eat crab, I feel sick afterwards.
Or
Whenever I've eaten crab, I feel sick.
The last tense I'd choose would be the progressive, unless I wished to imply that the sick feeling comes over me while I'm having the meal.
Whenever I'm eating crab, I begin to feel sick.
An important thing to bear in mind here is the sequence and nature of the actions. As we can see from the last example, the sick feeling can be an incipient feeling (begin to feel). In the first example, using the present tense, we have habitual action; to make abundantly clear that the sick feeling is the result of eating crab (and not the reason for eating crab), we use afterwards.
Whenever I eat chicken soup, I'm feeling sick.
In the case of the chicken soup, it is eaten because you feel sick. The implication is that you don't normally eat chicken soup, and treat it as a remedy.