I'm an 18 year old active female student and have been mostly paleo (exceptions of potatoes, quinoa, and some legumes) for about one year. I've felt amazing, better than I knew was possible. However, since I'm young and have a fast metabolism and no gluten, dairy, etc sensitivities, I've slacked for the last few weeks and let myself enjoy the occasional pizza slice or ice cream. Well, I've reacted very very badly - nonstop stomachaches, tiredness, headaches, and digestive/bowel changes. I could eat things like that before I went paleoish with no problem at all. I just don't understand why, as a healthy young 18 year old, I wouldn't be able to tolerate these things even occasionally?