She lost 1.5 lbs in 3 days wow - that doesn't seem right? - was that on the same scale each time?
Was she born premie - ?? I only ask because it seems that she was being seen/weighed quite often.
SInce she's getting weighed so much - can they weigh her before a nursing, then weigh her after?
Have you considered/looked into reflux? Babies suffering from reflux do not like being flat.