Are you eating things that are hard to digest? It's a shame to eat healthy and have this happen but it happens to many people starting in middle age. You, almost, really have to eat like an old person. Things my Grandmother would not eat (or knew she would suffer if she did ...) nuts, broccoli, cauliflower, most vegetable if they aren't cooked well, beans, garlic, spicy foods, almost anything with a lot of fiber.
Eat like an old person for a few weeks. Look up what people eat when they have stomach cancer (no, you don't have stomach cancer, but start there). See if that helps.