18 wks pregnant... have had stomach bug past 2 days vomiting and nausea no diarrhea or fever or other Sx. Should I asked to be tested for listeria?

Contact your OB MD. You need to ask this question of your treating OB MD who will know your history and physical exam results and can manage the continuing care.
Maybe (not) Listeria is pretty rare. It is spread through things like deli-meat, unpasteurized milk or contaminated vegetables or cheese. If everyone with a stomach bug was tested, there would be a lot of needless tests. Discuss it with your Ob provider to see if your symptoms and exposures warrant testing or treatment. Best wishes.