Breastfeeding and not gaining enough weight?

Took my son for a reweight check today and my dr was not satisfied with his weight gain. She does not think he's gaining enough weight. 
When he was born he weighed 8 lbs 9 oz. when we left the hospital he was an even 8 lbs. 18 days later he has only gained 2.5 oz. I have been strictly breastfeeding him and have only supplemented a couple times due to waiting for my milk supply to come in. So after talking with the pediatrician today she came to the conclusion that she thinks he has acid reflux. Not sure how she came to this conclusion since he has only spit up twice his entire life but I am extremely frustrated. She prescribed him Zantac drops. 
Has anyone else experienced this or had issues with their baby gaining weight?