Hard Poops

My daughter just turned one and we have started feeding her milk. Not a lot. Mostly water all day but, maybe a couple of ounces in a sippy cup during the day then mixed with her formula at night. We are at the bottom of the formula box so we are trying to get rid of it. Anyway, since then I've noticed that she has a harder time passing a bm. She still has one regularly every day but it's always hard. Could it be the milk? Do I need to give her more water during the day? More fruits and vegetables? I could use some advice. Thanks!