Slow weight gain :(

My daughter was a bigger baby at birth, 8lb15oz. I had A LOT of fluids pumped into me during labor, so I don't know if that effected her birth weight. She got down to 8lb5oz on day 8, but then she started gaining, just slowly. By day 14 she was 8lb11oz, and finally made it back to birth weight at 3 weeks. My ped was initially supportive and understanding, listening to me, but at 3 weeks he started using scare tactics to try to get me to supplement. I'm a third time mom, very experienced at bfing, but I can't figure out why she's not gaining quickly? She has no other signs of a problem. She nurses very frequently, acts satisfied (at least for a short while lol), has plenty of wet and dirty diapers, and when she is awake she is alert. All the signs that she is doing well, but she isn't gaining an ounce per day. I'm now (per ped's insistence) pumping after each feed and giving that to her as well. She only sometimes finishes it. At first I was getting 2-3oz each pump, but now I'm lucky to get 1.5 Oz. It seems unlikely that my supply would DECREASE when I am nursing and pumping more frequently (and VERY frequently, between the two it's almost every hour!), so now I'm wondering if she's just finally becoming more efficient at emptying the breast? Despite my experience, I've never had this problem before, so I'm very grateful for any advice!