Supply issues

Have been ebf since birth. Noticed she didn't seem to be gaining weight when she was 4 months old. Took her to the doctor and she recommended supplementing. So i have been giving 2 ounces of formula after I breast feed. Not using breast milk cause all I pump goes with her during work. I drink tons of water, have a lactation smoothy and I don't know what to do! Really hate giving her formula