EBF baby not gaining weight

My baby girl is almost 5 months old and she weighs 6.200 kilos (13.6 pounds) while her height is well above average. The doctors are starting to be concerned about her weight (she only gained 200 grams in the past month) and advised me to start her on cereals / solids now but I was planning on waiting until 6 months. Any other moms in my situation?