I would be concerned now.
My son is on the other end of the chart - 98th percentile height & weight - and if he lost 2 pounds, I would be concerned. Not panicking, but certainly concerned.
I am tiny. Always have been off the chart tiny... If I lost 2 pounds now, I'd get concerned and consciously try to eat some extra calories.
There are a few other things that come to mind:
Make sure you aren't accidentally sabotaging her appetite with to many carb/sugar snacks. We did this with my son at about this age.
If she just started walking, the extra activity can cause a slight weight loss. Or re-distribution... I've known a lot of babies to "slim down" when they start walking - only to have a growth spurt shortly thereafter and be walking around in pants that suddenly look like waders!
With the increasing warm weather, make sure she's getting enough liquids that she's not losing her appetite due to minor dehydration. If she doesn't want water - try watermelon!
I would also recommend going to the Dr. again - or a different Dr.! That far outside the growth curve warrants additional research, just in case it isn't just being a cute petite little thing!
Hope that helps - or at least gives you a starting point for conversation with a doctor!
T.