We went through a similar situation with my son when we started introducing solids to him arond 6 months old. Sometimes, he would seem uncomfortable, while other times he seemed fine. His pediatrician at the time recommended the "Ps"... peaches, prunes, peas, papaya, any food that starts with a "P." She said, "The P will make him poop," and I'll be darned if it didn't work! Not only did it expand his tastes (and my horizons), but it helped. Oatmeal cereal rather than rice cereal helped as well.
However, keep in mind that your breastmilk is a natural laxative for her, and it's perfectly normal for her to go several days between poops... but only if she's comfortable. You say she's writhing like she's in pain. Does she cry or show other signs of discomfort? If so, I'd try the foods that will help her go and avoid the foods that will stop her up (rice cereal, bananas especially!).