Dresses being "flowy, free, unrestrictive" depends a lot on the material. If it's a stretchy material, it'll be a lot more comfortable to move around in than say, taffeta, or other stiffer materials. Also, ease of movement depends on the style and length of the dress. My wedding dress is fairly comfortable, but it touches the ground. It is VERY DIFFICULT to walk up or down stairs and not step on the hem, potentially tripping me up, even when holding the hem up a bit.
Full skirts allow more range of motion than a tight skirt, but then you have to contend with the yards of fabric swirling about you, tripping you up, especially if they are heavy fabrics. Light fabrics (like those used in ballroom dance gowns) are fairly easy to run/dance/walk in freely.
Your character will have to be careful if there are multiple layers at the bottom of the dress--if the dress is a little too long, then she could accidently step on a layer and rip it--not fun.