In my opinion they take way too narrow of a viewpoint on CF weight. You all have spoken about it so well here. Family history, growth spurts and well as slow periods are all so normal in childhood, individual make-up, ect. They are not respecting you as a mother because they are not taught to do so. They have grown so accustomed to thinking they are our saviors that they simply can't see that we are the experts in our children's lives; they are simply part of OUR TEAM. Rarely, you will get a doctor who treats you with respect or sees you as a partner. Most of the time it us constantly training them in the way we would like to be treated. Sometimes it changes the doctor patient relationship, other times it is a lost cause; the ego is too large! If the latter happens get different people involved. No one says you have to employ her as your nutritionist! Boot her off your team. FIRE her. I'm sure you can come up with better ideas than PEDIASURE..JEESE is this their answer for everything. This stuff is crap.
I will say this, when I experience this attitude I begin to empower myself. I begin to look at how I can change things (if they even need to be changed) on my own with other techniques or professionals who have a broader more evolved perspective on human beings. They told me my breast milk wasn't good enough for my child, they had something better. I didn't buy it, I researched my options, employed the support of my pediatrician, my midwife, my la leche league and came up with a plan to give it a shot. I was right...he just needed more time than they were willing to give him. He grew well once he caught up from being undiagnosed for so long.
You are her mom, don't give up your power. Be strong in your intuition, try other more creative solutions if you feel they are necessary (tons exist), scratch the pediasure and OTC calorie drinks (they all suck) and teach them about the strength of a mother's love and care.
Sorry, I have just felt what you are feeling and it just gets me going.