underweight toddler

Good morning,
I have a 15 month old who seems to be continually dropping in her weight percentile.  When she was born she was in the 75 percentile for weight and now she is below 10.  Around 3 months she was actually losing weight and was diagnosed with acid reflux, she was put on Pepcid which really seemed to help and she started gaining weight again and then was following the growth curve between 25-50 percentiles.  Between 6-12 months she dropped to a range between the 10-25 percentile and stayed on that curve pretty consistently during this time.  We just went for her 15 month check and she has only gained 5 ounces in 3 months and she has fallen well below the 10th percentile.  (Her current weight is 19lbs 13 ounces and height is 30.25 inches). The doctor did some blood tests to check for diabetes and thyroid and both came back fine which is such a relief.  I just don't understand why she is not gaining weight.  It seems to be that every time she grows in height she drops more in her weight percentile.  Her height percentile has been very consistent between the 50-75 percentiles.  But the more she grows she seems to drop in weight.  
We have to go back in one month to have a weigh in but in the mean time is there anything I can do to beef her up a little?  She gets a very balanced diet and I am still nursing her because she won't take a bottle of any kind and she won't drink milk from a cup either, I am trying everything.  Could there be another reason she is not gaining weight?  I feed her until she won't eat anymore and she is still not gaining.  I am feeling very worried about her. Please help.