Civiking, go back to your doc and tell him about the redness/flushing of your face and neck. That symptom could indicate something other than a case of food poisoning.
In addition, if the lethargy is quite severe after eating, you might want to ask to have some food intolerance testing done. One food group - among others - that can result in lethargy (among other symptoms) is gluten (wheat, rye and barley). You may want to check to see if any family members have been diagnosed with celiac disease or non-celiac-gluten-sensitivity since your symptoms could place you in that/those categories. And yes, problems with gluten and other foods can 'suddenly' seem to appear.
any type of food poisoning can have a lasting effect on your digestive system. The digestive system is a balance of good and bad bacteria. When you get food poisoning it throws that balance out of whack. My suggestion would be to start taking a probiotic and eat some yogurt. See if that helps. Also try and stay away from food that are hard to digest.