My little girl has been a bit constipated latel...

My little girl has been a bit constipated lately so I thought I would go do a weigh,feed,weigh. She had only drunk 120mls!! That is like half of what a 7mth old bub should be drinking. When I got home (less than 1 hr later) I made up some formula and she drank 150mls. What do you think I should do? I am not 100% concerned if I have to give up breastfeeding except for the cost of formula and the inconvenience of it. Although I would love to have my body back to myself. I really don't know what to do. She hates drinking water too...it is an ongoing battle.