Breastfeeding and Supplementing

So I have been breastfeeding and baby latches like a champ since day one. She is a slow eater but eats well. I make plenty of milk and can pump between feedings and get 4+ ounces. However, when I got re-admitted to the hospital for postpartum preeclampsia and was on a strict fluid restriction, my daughters weight took a hit.  My daughters doctor said it might be good to give a meal a day of formula a day to help her catch up. She will drink the formula no problem but gets gassy.