My 18 months old is at 9kg and this was her weight since 6 months old. Is this something I should be worried about? She is eating well and growing well just no weight increase.

Like what Hui Qun has mentioned, most babies tend to slow down their weight gain at the 6 months mark. I personally wouldn't worry too much about your baby's either as long as she's eating well and as Hui Qun said, hitting her milestones accordingly. Babies don't necessary have to be of the same size as other babies because well, they come from different sets of parents so genetically, they're all different.