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.

Babies double their birth weight by the time they complete six months. However, after that the weight increase slows down a bit. If your baby is active and eating well, there's nothing to worry about.Don't worry and just keep monitoring your baby's weight and show it to the doctor the next time you visit him to be on the safer side.