The first month babies weights fluctuate a GREAT deal. If you weighed her 5 times a day... each time would be different. Often by more than half a pound to a pound. (Depending on when they last ate, peed, & pooped). My own son was an "off the scale" eater (avg of 20oz per feeding every 2 hours... so he'd actually have a 2lb+ variance during different times of the day). But it's a big part of why babies aren't usually weighed every day unless they're deathly ill. By being weighed at the set appointments it's more possible to see the actual progression. Also, babies DO actually cycle with weight loss when they start getting more active: typically at a couple weeks (when they start doing a lot of large motor arm and leg waving), a couple months (rolling over), crawling, & walking.
Most babies will also eat whenever food is offered them... but they won't eat nearly as much as they will otherwise. An ounce or two instead of the more normal 4-8oz. If mom is nursing, baby may very well be snarfing quite a LOT (hence the longer sleep times). But the only way to tell is via the super spiffy baby scale that lactation consultants and the NICU use (the kind that measure out to hundredths of grams and are sized for large loads and calibrated to average out the wiggles which can increase or decrease weight depending on the type of motion).
If the baby is in every other way thriving (alert, hydrated, good color, active, etc.) I personally wouldn't be worried.
COULD there be something wrong? Sure. But simply losing some weight (if that's all there is), wouldn't worry me. Lethargy (not doing the arm and leg waving, but being ragdoll limp), glassy eyes, not being alert, bad color, dehydration (few to no wet diapers)... those would be far more "Yikes" to me since she's already started to show a pattern of lose, gain substantially... I would bet that pattern will continue (it's a pretty common one... lose gain substantially, lose, gain substantially). If she'd never gained at all I'd be very concerned.
My personal "when in doubt" however, is to go to the ped.