Allergies

Sometimes you can develop transient allergies, or new allergies. Particularly to foods you eat all the time.

That you are eating exactly the same foods isn't necessarily armour to allergies. A limited or very repetive diet can actually trigger you to become allergic to a food.

For example, my father eats carrots everyday. Every now and then he develops a sensitivity to carrots for a while. Then it goes away. I develop a reaction to strawberries every couple of years, then it's gone. You'll find that an allergy in one country is common, and very uncommon in another. Usually in the one it is common, the food is common.