I think you need to consider that she hasn't lost weight - she has consistently gained, just not quickly. I think that says something. If she was FTT I would think that she wouldn't be gaining at all. You have said she is happy and healthy - not lethargic. Also I think a mum is the first person to get an idea that something is wrong, not a dr. You are with her all the time, if something wasn't right you would notice! And JFYI my youngest is constantly sick (ear infections every fn) so is on AB a lot and when they irritated her tummy she stopped eating, so if there was something wrong with her tummy I would think she wouldn't want to eat, which isn't the case!
I hope the MCHN helps to calm your fears though but I say if mummy instinct says she is fine she probably is!