Lil, some babies are just lighter than others. To help increase supply, as well as what Kel and MG posted, you can use your nipples as a dummy. The more they suck, the more milk you produce.

Our health visitors tell us solids from 17 weeks, don't leave it any later. I ignore them; DS is still fully breastfed and I aren't willing to introduce solids just yet. It's completely against WHO guidelines, but when I pointed this out I was told that was just so "people in Africa didn't do it too early to be Western... it's fine for our babies." But if a baby grabs some food, puts it in his mouth and enjoys it, maybe it is time to start weaning irrespective of age!