This sounds like a claissic case of "catch down" growth: ie child born above its genetic weight (due to the excellent nutrition provided by its mother ) and then having to adjust to its "natural" curve. So of course she is ging to drop down the curves.
Ds was born on the 91st centile, dropped (IIRC) well over a pound and then took c.7 weeks to regain his birthwight - so of course dropping through the centiles as he did so. He never actaully lost weight - just gained it extremely slowly.
Eventually he started to follow a "curve" jsut below the actual centiles, before starting to creep back up until the charts and then follow the 25th for c.6 months and then on up to the 50th, where now aged 7, he remains.
This is exactly as predicted by the consulatant paediatrician who he saw (referred by the breast feeding counsellor midwives, who weren't exactly worried by ds, but were just covering their backsides). He saw a happy healthy baby in fornt of him.
Ds' rate of weight gain never changed, even when after topping up with EBM and then also giving every second feed as EBM, so that I could see he was having plenty.
I was fortunate to get lots of support - but also made sure to avoid HVs and just saw the breast feeding counsellors at the maternity hospital who ran a support group.