DD1 had gastro at 12m and lost a lot of weight hitting the 3rd percentile at 12m. She only hit 10kg at 18m and had to be checked at 15m and 18m to ensure she was gaining back the weight. I would be weighing her regularly to see if she is gaining it back once she is well and only get concerned if she isn't. FWIW - DD1 is still tall and skinny!