Of course I also don't know you, but I just wanted to say that in my situation, as well, my LO at 18 months was 21 pounds, and 33.5 inches tall. I don't know her exact weight now, at almost 2, but its around 23 pounds. She has grown a lot in height since 18 months- I don't know exactly but if I had to guess it would be about 35 inches now. She's tall and skinny, and always has been. She has gained a little consistently but is always getting taller fast. If you look at her relatives, many are the same way.
When I get her in for check-ups, they once mentioned giving her more dairy, but then when I said I still BF, they said that has more calories anyways. I never changed much, just offered food, but she never has eaten much. We also don't eat much "junk" food...so maybe that has to do with it too.
Lots of babies are smaller, of course don't ignore what the doctor has to say, but also realize that smaller babies/kids can be fine too. Maybe see how things go and get another opinion. You wouldn't want to miss an issue by just doing nothing, but I think there are also a lot of doctors who don't see many BF kids who eat well and are small.