How do I get chronic flatulence if I already stopped drinking milk?

Maybe not milk. Lots of things cause flatulence. Some are foodstuffs: lactose, fructose, sorbitol (in sugarless gum, candy), beans/legumes, veggies like brocolii, cauliflower, onions garlic, carbohydrates in starch, gluten in grains, carbonated beverages, coffee, alcohol, swallowing air, smoking, as well as intrinsic GI disorders. Do a diet diary, with symptoms; bring to your doc; figure out what's doing.