My baby is 13 mths, 8 kg and 74cm. His height and weight is also below 50% percentile. He is also hitting developmental milestones. I used to be very worried but I also dont't want to force him to eat and drink if he is full. I dont't meals to be undesirable to my boy.
You will know if your girl is teething if you notice that she is drooling excessively. Drool is usually one of the clearest tell tale signs that a baby is teething. More so than loose stools or loss of appetite because those can be due to other factors than teething.
You can try to change the teat size so that she can drink more and faster if you suspect that that is one of the causes of affected appetite. My boy can be like that. If he takes too long to finish up his milk, he gets bored and doesn't want to drink anymore.
My pediatrician also gave him some multi-vitamins which would also cause his appetite to increase. So far it only worked slightly.
I know kids who're 19mths but still 8-9kg. They're hitting developmental milestones, which I feel is most important.
Your girl could also be small if it's in your genes, ie, you and your spouse were small as a baby and todd.
For me now, as long as my kid has a healthy diet and is hitting the milestones, I'd be happy enough.