One year old not gaining weight!

We just went for his one year check up and since his 8 month he hasnt gained a thing! he is 18.4 lbs and looks healthy! I just thought he was smaller for his age ( he wears 9 mo clothes) but the pediatrician made me feel horrible like I am not feeding him enough! He still nurses and eats some grown up food but is so picky! She said that he is on the line of being malnourished on the chart.  Has anyone had this happen? Is it just a stage? What did you give your LO to bulk up? I also heard that some doctors use outdated formula fed baby charts.. any advice or experiences?!