Is dd happy, healthy and alert? If so, the centiles don't really matter - all they are are averages, and some babies don't follow the "average" growth curve.
My ds was also born on the 91st centile for weight and head circumference (I think he was just 50th for length, but I can't really remember), but then progressively dropped through the centiles until he was underneath the charts!
Throughout this this time, he was happy, healthy and alert and (to my dh, more importantly) my dad, who was a paediatric radiologist, was never worried.
Anyway, I got loads of support from the midwives/bf counsellors at the bf support group at the maternity hopsital and they got me feeding him extra EBM, making me wake him up for feeds and so on, but when he still didn't gain weight (although he never LOST weight), referred him to the consultant paediatrician. He took one look at this manifestly healthy baby, told me to stop faffing about with the hassle of expressing (I didn't, as it was useful as I was going back to work at 4 months) and was more interested in knowing what my dad (who had retired) was up to.
At about 3 months he started to creep back into the charts, chuntered along on the 2nd centile for a while and then gradually moved up to the 25th.
I exclusivley b/f for 4 months, before beginning to wean him and went on to fully b/f for 13 months.
He's now 2 1/2 and I don't have a clue where he is on any of the charts. he's a happy healthy boy and that's all that matters. At his 2 year check, he was roundabout the 50th centiles for everything I think.
How big are you and your dh? Dh and I are both small and were, when we were young at least, skinny, so it's no wonder that ds followed the same pattern. Weight at birth is supposed to be more a reflection of maternal health and it's only AFTER birth that the the genetic influences come into play more strongly. I suspect I was borderline diabetic as well - I kept on getting sugar in my urine, but not in my blood.
I suppose the key question to ask is: if you weren't measuring her, would you be worried? Trust your instincts!