Time for an angry rant

Ok, so I know that nobody else thinks this is quite as annoying as I do, but I need to get it off my chest.
As you probably already know, Ivy has a nurse that comes to our house once a week to weigh her and check her oxygen levels to make sure she progresses well before her next surgery. Every week Ivy is supposed to have gained 5 to 7 ounces and her oxygen level needs to be between 75 and 85 to be considered thriving. Her oxygen level is always great, but for the last month or so she hasn't gained enough weight, it's always between 1 and 4 ounces. The nurse made me put her feeding tube back in about a month ago, the first week she didn't gain 5 ounces. I took it back out about 10 days later because I disagreed with her (Ivy didn't gain more weight with it in). It is known that kids get oral aversions when they have a feeding tube for too long, and I think the nurse didn't know what else to do, the tube was the easy solution. It turned out not be a solution, just a giant pain in the butt.
Last week when the nurse tried to tell me to put the tube back in I refused. I told her that it wouldn't help, and we needed to find a better solution. I suggested that one of the reasons she failed to gain weight could be that she spits up quite a bit. The nurse agreed and sent us to our pediatrician to see if he could prescribe her a second reflux medication (she is already on zantac). Our pediatrician decided to look at her overall progress instead of week to week. He said she is still within her growing curve and sees no reason to be concerned. She has gained about 2 1/2 pounds in just under 2 months, and that is fine. So he didn't prescribe the other reflux med because he thought the risks outweighed the benefits. He suggested we feed her less formula(45mL), every hour and a half, instead of the full amount(70mL) every three hours to cut down on her spitting up. So that is what we have done (except at night, because really, every hour and a half, I would probably murder someone).
Nurse Nancy came yesterday and again Ivy gained only one ounce. I have really come to dread the days that she comes because I know Ivy didn't gain enough (I weigh her myself), and Nancy is going to give me that look, like your not doing enough to make her gain weight. She asks every time, "How much are you feeding her, and how often?" Even though she knows the answer already, and then she looks at me like she doesn't believe me. It's so frustrating, I'm not lying, I feed my child, all the time it seems like. I can't do anything else, because I am always feeding her!
So now I ask you, (those of you with children)how much did your babies weigh at 4 months? Do you think your kids gained 5 to 7 ounces EVERY week? Is it possible that Ivy is just small, like my boys who are both in the 10th percentile for weight. They weren't like that when they were babies, but they are now.....does that matter.....I don't know what to do. I'm frustrated and feel like I've tried everything. If you have any suggestions please let me know, I could really use some good advice.
And FYI - In other areas Ivy is right on schedule, she smiles, coos, and laughs. She is becoming an expert holding her head up, she swats at toys dangling above her, etc. I can't imagine that anything is really wrong when she is doing so well in all other aspects!