Penny - If a food item gives you a bad reaction, do not eat it.
These days a lot of people have problems with food. It doesn't need to be an exotic mushroom, gluten in wheat and lactose in milk are problems for many people.
It has long been known and documented that some people react to various edible mushrooms. Some form of gastric upset is the common reaction, although more extreme ones have been noted. If you are one, avoidance is the sensible option.
As for telling your niece, simple say what you said in your first post - They were delicious, but unfortunately you seem to be allergic to them.
Quack!
Geoff the Duck