Can someone be allergic to a food sometimes and other times, not?

Usually not: But if something you are eating has multiple ingredients (an omelet, for example) you won't know. Only by eating the food itself, alone, can you tell. You also need a healthy bacterial population in your gut for your immune system to react properly. Add fiber (legumes, veggies, whole grains) and probiotics if you are unsure or have taken antibiotics or Prednisone or are very stressed.