There's nothing wrong with your milk supply or your dd's growth. Seriously. Look at her. Does she seem healthy and thriving? If you didn't have any numbers to look at, would you think she was okay? Do you really expect her to stay at the 90th percentile her entire life? That means she would be heavier than 90% of the other children. With babies that's not significant, but obviously at some point she's going to "slow down," unless you or your dh are exceptionally large people genetically?? Also, keep in mind that if you had IV fluids in labor, her birth weight was probably inflated due to excess water weight.
My son started at 7 lbs 8 oz. At four and a half weeks he was at 8 lbs. 11 oz. That's really not much of a gain for a month... and he did not stay on his curve. But he grew 3 inches, and his head circumference increased appropriately. He is fine, and my milk supply is fine. Our family doc actually joked with us about starving him... knowing he is perfectly healthy. So, my oldest and youngest are long and thin. My two middle kids were at about the 50th percentile for birth weight, but then jumped up to the 99th percentile on the charts... and stayed there for about a year. Then they stopped gaining weight, and just grew up and thinned out, and went back down to the 50th percentile, where they've basically stayed. They're all perfectly healthy. Kids all have their own growth patterns.
If you need more reassurance, plot her out on the WHO growth charts, which are based on multicultural breastfed babies in thriving environments. Here's the link below. Click on the "girl's growth record" to download the charts. The charts your doctor is using is based on a small group of formula fed babies. Babies and children do not grow on a predictable "curve." There are spurts and plateaus. The CDC has actually adopted the WHO growth charts, but they haven't made their way to doctors' offices yet, and most are still using the old charts. Or, get a second opinion from a lactation consultant or another doctor.
Your dd is still gaining. She is fine. Find a new doctor. Even if she was having growth difficulties, the advice you were given on how to remedy that is completely illogical and counters research. Cosleeping babies sleep lighter... which is a good thing, because they feed more often and it also protects against SIDS. Check out some of the research by Dr. McKenna.
Formula is just a bad idea in general.