Weight support

Our little one was born at 6 lbs. 15 oz. just over three months ago. She is currently 11 lbs. 8 oz. and using Similac 360 ready to feed. I’m getting concerned that she’s not gaining enough weight… Has anybody else gone through this? She’s drinking 28 ounces a day and has stopped spitting up as we have spaced out her feedings so I am worried about simply increasing her amount of formula a day…