Has she been measured in length? Head circumference? Some babies grow more in length and head circumference and it takes a great deal of calories and fat to grow long and for their head circumference to keep up with its centile. The three weeks my second youngest had the worst weight gain and even lost 4oz, he grew 7.5cm in height (an inch a week!) and jumped a centile in head circumference. He went from having to have massive turn ups on his trousers to not needing any turn ups at all.
People are crazy. My youngest is between the 25th and 50th centile (born nearly on the 91st) in weight yet people keep saying how massive he is and am I sure I'm not feeding him formula on the sly! Yet my second youngest is quite big for his age and I always get comments as to how small and slim he is xx