She could have her own growing curve. I'm assuming she is your first child... do you have growth records from you or her father when you were babies? That could shed some light on whether this is just a growth pattern specific to her. If she's reaching all of her milestones on time, that would make it more likely that this is just the way she grows.
All of my kids spent their entire baby/toddler years at the very bottom end of the weight charts (I even had a pediatrician threaten to call Children's Services unless I fed her ice cream to get her weight up at 3 months old - I didn't feed her ice cream, I switched doctors and she's now 16 and has a healthy weight). It's just the way the babies in my family grow - every baby in our family has done this and turned out fine! They just have growth spurts (in height and weight) at different times than most babies - and the charts are based on most babies. I would also ask which weight/height chart your doctor uses. Older versions are based mainly on formula fed infants who gain more weight faster than breastfed babies. And, if they are using more modern charts, you need to remember that most babies do switch to formula well before 12 months old and that also skews the charts in favor of more weight gain. In fact, according to WHO's height/weight chart (which is said to be based on breastfed babies only), your daughter ranks around the 30th percentile in weight. Not bad at all!!! Try not to worry, if she is reaching her milestones, she is most likely fine. My 12 month old is at 3rd percentile for weight (according to her doctor's chart - more like 10th according to WHO's chart) and she's reaching all of her milestones very early. Her doctor just tries to encourage more food - but isn't worried. Plus, if your daughter reached gross motor skills exceptionally early, that would change her curve as she burns more calories earlier in her life than most babies. Ack, sorry for the jumbled mess of information. I was just trying to get it all out.