I think our DD was around the same weight but in the 25% percentile. 19 lbs at 15 months doesn't seem like it is that low. Our DD was around 15 months old when we turned the car seat the other direction because of her weight. Have you tried those pediasure drinks?
Today she went for her 2 year check up and she is in 50% for weight - she is 27 lbs and 36 inches tall. She gained 5 pounds in 6 months and grew almost 4 inches.
She loves cheese!