My baby is 8 months and weights only 18lbs

Should I be concerned ??? She eats every 3 hours still and eats plenty of solids drinks plenty of milk but she just won’t gain weight??? She gained only 1 pound in 2 1/2 months. She’s tall for her age but I’m concerned she’s skinny. The doctors said she’s just tall and skinny is her body type but I’m worried. How much do your babies weigh ????