I didn't put any weight on when I started lithium, but a few other meds were bad for it (especially the zoloft + zyprexa combo, which was actually before the lithium). At one point, I'd put on close to 40 pounds. Wasn't really hungrier or eating more, it just sort of happened. Diet and exercise are key to fighting weight gain from meds. Mostly kept the extra weight for several years, dropped it pretty much effortlessly when I started on risperdal.
Your mileage may vary, however.
If you're serious about another child, you need to have a talk with your doc about meds during pregnancy, if you haven't already.