I agree that its probably just a pregnancy thing. I've had a very ... unwieldy GIT my whole pregnancy!
But even if it was a bug, the chances of it being listeria are SOO low. Much more likely to be garden variety E.Coli or something like that!
Even women eating soft cheese and sushi and hotdogs have a very low chance of picking up listeria, and my midwife said that in 30 years of practicing, she's not once seen a case of it!
If it makes you feel better, there have been some studies showing that maternal exposure to bacteria decreases the child's risk of allergy E.Coli is our friend haha
The only thing to watch out for is fever
Hope its all cleared up for you now