i lost 3 kgs ..i have been eating less than 1200 calories but today wen i got on..i gained a 1 kg

Its not a true gain. Weight fluctuates all the time due to sodium, time of month, and other things. Be patient and keep at it. It will be off again, and then some. Promise. Just keep at it. Don't give up. That is the biggest reason people fail is because they give up.

And unless you are very short or bedridden you shouldn't be eating less than 1200