Actually, you burn more calories breastfeeding than you did while you were pregnant, so while you do put on some extra padding to ensure there is sufficient fuel to burn to make milk, (which is actually stored in the 'saddle bag' area), if you are exclusively breastfeeding and eating about 2000 calories a day, and providing you gained 35-40 lb. in pregnancy, you will probably burn all of that off in about 6 months just from nursing (also depending on activity level, etc).
However, your question is about weight gain after breastfeeding stops. Yes, it is common to gain weight, if you do not burn those extra calories you had been burning in lactation in some other way, and you continue to eat the same amount of calories.
The bummer of that is, that in order to burn the same amount as when producing milk, it would take 2 or 3 hours a day on a treadmill if calories aren't cut! If I had known that, I'd have kept lactating and donated to a milk bank!
K. Wildner, CCE,CHt, HBCE
www.kimwildner.com