Hi first of all u already reduced 12kg in 4 months its a good thing....wether u hv deliberately reduced weight by less food intake or naturally lost is not known.So take it slow.Baby needs atleast 6 months exclusive breastfeed.So first eat healthy and nutritious food. U can eat Non Veg if u r a non vegetarian.or else take plenty of fluids milk and bread ...and green leafy vegetables which will improve lactation.
Coming to weight loss u can start doin tummy exercises now....wether belt will reduce the tummy or not is not evidence based medicine but it prevents backache and abnormal position.
Next Steps
so start walking ...tummy exercises slowly and eat nutritious balanced diet.
Health Tips
It took 9 months to build up the weight so it can be lost in the same time.Don be in a hurry.Take it slow...u will definitely achieve ur ideal weight.