i had the same problem with my mirena. i had it removed on friday, after 5 months because of such severe side effects, and i have "pooed" more in the past 4 days than than i think i did in a week when i had the mirena. on the bright side, about 75% of my other symptoms have all but disappeared- the foggy mind, bloating, headaches, shooting pains in my sides, etc. i'd go with the fiber one cereal idea. they also have tasty chocolate bars made by fiber one that i ate for snacks on those bloated days.
good luck!