Right before giving birth, i weighed 181 lbs. after leaving the hospital i was 166 lbs & now i weigh 179lbs at 2 months postpartum. i am breastfeeding & supplementing with formula. what could be causing rapid weight gain postpartum?

You likely: have insulin resistance/carbohydrate sensitivity, causing the weight gain and fluid retention. This can lead to diabetes. You need to be tested for insulin resistance, with a 3 hr glucose tolerance test, and insulin levels. Talk to your doctor or specialist in Diabetes/endocrinology. Thyroid abnormalities also could be a cause, as well.