Well, skin p-r-i-c-k test.
Allergy to peanuts/tree nuts are common. Face flushing, diarrhea, difficult breathing, "hives" or eczema, are possible additional signs of allergy. To be sure if yoa are really allergic to peanuts (or/and other nuts), you should have a skin ***** test. It's easy, try to arrange this.
Allergy can appear at any age.