Fake Allergy
As a kid, you’re bound to believe anything your parents tell you. When this OP’s mother told her that she was also allergic to pineapples just like her, she never questioned it. A lot of allergies can be genetic and she had seen the reaction her mother had to it.
It was only when she was in her early 20s that she realized that her mom had never actually confirmed whether the OP had the allergy too. She ate a piece of pineapple and was just fine. Wow, that’s just crazy! She missed so many good years of pineapple eating and it was all because of a lie.