I never had food allergies as a child, but I seem to have them now as an adult. Is this common to happen?

Adults get allergies. People are born with the genetic predisposition to develop food allergies and they can occur at any age. Food allergy reactions usually consist of some/all of following: itchy lips/mouth/throat, nausea, vomitting, abdominal pain, diarrhea, hives, lightheadedness, chest tightness/wheezing. These symptoms occur each time a bite or sip of the offending food is eaten within 2 hrs of ingestion.
Not unusual. Many food allergies develop in adults, particularly shellfish allergies, but also peanut and tree nut allergies. There are also other immune-related stomach problems that can be related to certain foods.