All i did was exclusively breast feed my baby till he was 6 months without even giving him water . That helped me loose my weight. Breast feeding alone consumes about 450 calories per day. My baby is now 16 months and I am 5'3" and weigh 50kg. I think this is reasonable. :-)
In your case, baby is already 5 months, so i think light exercise with proper diet would do the trick.