It takes a while for bacteria in your gut to adjust. Like a big part of why fiber is good for you is because it's food for the good guys there, but it takes a while for them to reproduce to exploit the new opportunity of increased nutrition. Thus radical changes in diet lead to the population ending up in an imbalance as some bacteria can't survive anymore and others have yet to take their place. So it's a good idea to make any changes gradually and only add fiber again when your gut has adjusted.

That said, you could also try eating fermented vegetables like kimchi and sauerkraut along with your fiber supplement. Fermented vegetables not only have fiber, but they also have lactic acid bacteria which eat fiber. You get a good population of fiber eating bacteria sooner.

Edit: Also fermented dairy can be good. Butter milk, yoghurt, kefir, sur and so on. You can make a pretty refreshing drink by mixing one part of greek yoghurt with one part of water. It's sweet and sour much like soda, but probably way better for you than that. If you use vegan dairy alternatives, there are also fermented options there.