Constipation

I think my little one is constipated. She is formula fed and has done fine on the same formula since 6 weeks (17 weeks now). She pooped every couple days (pedatrition said that was normal) and it was normal up until this week. Is there anything I can do to help her? Her poop isn't super hard but it's like clay. She doesn't seem to be in pain...only strains a little when pushing.