Chloe,
Sounds fine to me, as long as she is gaining some weight & is having wet & dirty nappies & is contented then she sounds fine!

I also am not an expert, but then tend to have growth spurts where they dain a heap of weight (for them) & then a bit of a stage where it sort of evens out again...

With my oldest she is almost 8 yrs & just 20/21kgs & is tiny.. I can tell she grows coz she goes thru a shorter chubbier in the face looking stage & then all of a sudden her pants look short! This only happens every 7-8mths!!! hee hee she is in size 4 clothes!!!

Sounds like she is just gonna be petite!