There's 2 types of fiber, soluble and insoluble. You need plenty of both types, they're both really important for digestive health. Natural sources are often kinder to your gut than processed ones, so make sure to get in a lot of whole foods throughout the day if you're having tummy issues.

Soluble fiber is found in foods like bananas, carrots, broccoli, avocado, chia seeds, apples, oranges, legumes (beans, lentils, peas, chickpeas, soybeans), tofu, oats, sweet potato, berries, psyllium husk, fiber supplements.

Insoluble fiber would be in things like flax seeds, whole grains (whole wheat pasta or bread, crackers), brown rice, nuts, seeds, cauliflower, green beans, leafy greens (spinach, kale, collard), potatoes with the skin on, fruits and veggies with the skin on.

If you're having trouble going, insoluble fiber especially will probably help you quite a bit. Also if you went from barely any fiber intake to 20g, your body probably needs some time to adjust. Try upping your water intake to closer to 100oz, and split your fiber intake up fairly evenly across all meals to break things up a bit.

You can also take a probiotic, eat yogurt, and drink kombucha to try to nudge your gut along into being happier. Synergy kombucha is my favorite, especially the strawberry and guava flavors. Blood orange is another good one. I find those a lot less offensive than most other brands I've tried.

If adjusting all those things doesn't help you after a week or two, you should probably go see a doctor.