Hey,
I understand your worries. My DD went from 10.28kg at 12 months to being 10kg at 19months (5ht percentile). I was beginning to worry, went to the hospital, saw my GP, and they both told me that as long as she stays above the 3rd Percentile, she is eating and has energy to run around, there is nothing to worry about.

My DD is very active, and a fussy eater. And she also comes from a petite female family from my DH side, not mine obviously, so I try not to worry anymore. Funny enough, when I stopped, she gained a kg, so finally she is off the 10kg mark and up to the 25th percentile, yey for her.

So as long as your DD is eating happily and active, I would say try not to worry. But if your concerns don't go away I would see your GP.

HTH