My DD1 had gastro before her 12 month check and when she was weighed was in the 3rd percentile (normally she was 15th-20th) so the MHCN got us back 3 months later to make sure she was putting on weight. At 15 month she was back up to the 10th percentile. When are are so little they can lose quite a lot of weight in a week from being unwell. It would be worth weighing her in a couple of months to see if she has gained. If she hasn't then look at your options. FWIW DD1 was only 11kg at her 2 year check so not much different to yours and she also ate fairly similarly to your DD.