I get stomach aches when I eat healthy?

So I’ve recently started eating a healthier diet with exercise. I’ve added more veggies (not too many) into my diet and have been eating very little fried foods, junk food, etc. Well since I’ve been eating healthier I’ve been getting bad stomach cramps/diarrhea almost every evening. And I know its not coincidence because this literally happens every time I start a healthier diet. It confuses me because I rarely ever feel this way when eating fast food/junk, but then when I try to eat healthy I get stomach aches? Is this a normal thing? Does everybody go through this? Thanks!