Baby didn’t gain expected weight

Today my daughter had her 2 month appointment. The pediatrician said that she is not at the expected weight but that she’s not underweight she should’ve just weight a bit more. She said she was born at 6 lbs 14 oz making her a 50th percentile baby but now she’s at 9 lbs 4 oz making her a 15th percentile baby and she should be in the 50th percentile since that is the percentile she was born at. She said she should be closer to 10 lbs. I went over her eating habits and she explained that I’ve been doing on demand mostly instead of timing feeds. I explained that during the day she usually eats every 2 hours and at night she has been sleeping longer stretches so I let her sleep. Last night she actually slept from 11:30-4:30 without waking up! She told me to feed baby every 3 hours and wake her up l when she’s sleeping. I explained that baby also spits up a lot of milk sometimes it looks like an ounce of milk. I explained that she uses me as a pacifier and NEVER unlatches, I always have to unlatch. She thinks that maybe instead of not eating enough maybe baby is eating too much so then she throws up and doesn’t get those needed calories. It was a bit discouraging as I have been told that feeding on demand is ok so I switched from timing it to on demand and now my fear of her not gaining weight came true. I’ve gone to 2 milk transfer appointments but it’s been at least a month since the last one and she did good on both so I know I have enough milk, plus when I pump it’s a lot and she spits up like an ounce sometimes. I set up an appointment for one month for a weigh in and we’re going to go from there. It’s also upsetting since baby has been doing better at sleeping through the night and now I might have to wake her up anyways. I do not know the purpose of this post is but I don’t have anyone to share with (besides husband) lol.