Which line was she born on and which one is she on now? Is she gaining weight, growing and bright and alert at times? How often are you getting her weighed? I think at this age the recommendation is once a month, but your red book will be able to tell you.
Why is it worrying? I don't understand. The weight charts are based on healthy babies, she is on the chart and has not dropped massively so that is a sign she is a healthy baby. Your baby has not read the red book and anyway, you can't feed a baby that isn't hungry. If there actually is a concern about the weight then more frequent breastfeeding might help, but if you are feeding on demand anyway then I don't expect you have much to worry about. How many times a day would you say she breastfeeds?