Baby stretching/pulling

Lately, my daughter has her legs stretched out and grips my breast while nursing. She grips very tightly so I try to hold her hand. She's not whining, fussing or crying at all but keeps her legs straight and stretched out. Almost seems as though she is trying to poop. I don't think she's backed up because she goes every other day like she has been for several months. And I don't think it's constipation since when she does go, it's not hard or solid. Anyone know what could be going on?