My little DD, is 1yr now and on her last weigh in a approx 3 weeks ago was only 5.9kg, she was born at 38wks and weighed in at 2.38kg then went down too 2.1kg and didnt get back upto her birth weight of 2.38kg until she was 5wks 4days.
How long is your DD now? Have you looked at her chart in the blue book to see if she is on the chart for her age in weight and length. My DD is with her pead as well as a dietician and have my DD as just over 1 1/2 kgs underweight for her length which is 65cm. (A length of approx 6mth old)
If your DRs are worried I am sure they would mention something to you, if you are worried about her, I would mention it to your DR.
All the best.