Average weight of 18mth old is 10.8kg, so your LO is on the lower end, but still looks to be a healthy weight. If you look in your red book are they drastically dropping %line or are they roughly following the curve?
My little boy is the same but over a longer period but he’s been constantly unwell and is always climbing and rubbing around paired with their growth slowing so I’m not worried :)
between 1 and 2 they only gain approx 5lbs over the whole year :) which is about 2.5kg, so given that she's been ill and potentially not eating as much during shorter spaces, still gaining steadily I wouldn't be worried :) also factors like growing, illness, mobility increasing can all impact it :) I would keep an eye out especially over the next month or so if she's now not poorly and see what happens :)